Cowboy Butter Portobello Flank Steak: 5-Ingredient Crockpot Dinner Step-by-step instructions

Before you start

Before you start, slice the flank steak against the grain into thin strips, about 1/2 inch wide. Thickly slice the portobello mushrooms and peel the shallots, leaving them whole if they are small, or halving them if they are large.

If you are making your own cowboy butter, prepare it ahead of time so it is chilled and easy to cube. Have the au jus mix measured and ready. No need to brown the meat or sauté anything—this recipe is designed to be as simple as possible.

Instructions

Layer and Season

1. 1Layer the Ingredients
   Place the thinly sliced flank steak strips in an even layer in the bottom of a 6-quart slow cooker. Scatter the thickly sliced portobello mushrooms and peeled whole shallots over the beef, spreading them out so they are evenly distributed.
   Look for The slow cooker insert should be about half full, with the beef covered by the mushrooms and shallots.
   Tip For the most tender texture, slice the flank steak against the grain into thin strips about 1/2-inch wide.
2. 2Add the Butter and Au Jus
   Scatter the cubed garlic-herb cowboy butter over the top of the beef and vegetables. Sprinkle the dry au jus mix evenly over everything. Do not stir—the layers will meld as they cook.
   Look for The butter cubes and au jus powder should be visible on top of the mushrooms and shallots.
   Tip If using homemade cowboy butter, make sure it is well chilled so it cubes neatly and distributes evenly.

At the end of this phase, the slow cooker insert holds an even layer of raw flank steak strips, topped with thick portobello slices and whole shallots. The butter cubes and au jus powder are scattered over the top, creating a neat, colorful arrangement. The ingredients are not mixed, which is exactly what you want. The butter and au jus will melt and blend during cooking, so there is no need to stir now.

Slow-Cook to Tender

3. 3Slow-Cook Until Tender
   Cover the slow cooker and cook on LOW for 6 to 8 hours, or on HIGH for 3 to 4 hours, until the flank steak is fork-tender and the mushrooms are soft. The butter and au jus will melt into a rich, savory sauce. Let it rest for 3 minutes before slicing.
   Look for The liquid should be bubbling gently around the edges, and the meat should shred easily with a fork.
   Safe temperature 145°F (63°C) internal temperature (steaks, chops and roasts)
   Tip For the deepest flavor, cook on LOW—the longer time allows the collagen in the steak to break down fully.
4. 4Check the Temperature
   Use an instant-read thermometer to check the internal temperature of the thickest piece of flank steak. It should register at least 145°F (63°C). If it has not reached that temperature, continue cooking for 30 minutes and check again. Let it rest for 3 minutes before slicing.
   Look for The thermometer should read 145°F (63°C) or higher, and the meat should be tender enough to pull apart easily.
   Safe temperature 145°F (63°C) internal temperature (steaks, chops and roasts)
   Tip Because the meat is in a moist environment, it will not dry out even if it cooks a bit longer than the minimum.
5. 5Rest the Meat
   Turn off the slow cooker and let the meat rest in the hot sauce for at least 3 minutes. This allows the juices to redistribute through the beef, keeping it moist and flavorful.
   Look for The sauce will settle slightly as the meat rests, and the beef will look glossy and tender.
   Tip Resting is essential for whole-cut meats—it prevents the juices from running out when you scoop the steak.

After 6 to 8 hours on LOW, the slow cooker is filled with a bubbling, golden sauce. The beef is fork-tender and easily shreds, the mushrooms are soft and have soaked up the buttery juices, and the shallots are translucent and sweet. The aroma is rich with garlic, herbs, and savory beef. The sauce is glossy and slightly thickened, coating the back of a spoon. Check that the beef reaches 145°F (63°C) before moving on.

Rest and Serve

6. 6Serve Over Mashed Potatoes
   Spoon the warm mashed potatoes onto serving plates, then ladle the braised flank steak, mushrooms, shallots, and plenty of the buttery au jus over the top. Serve immediately.
   Look for The beef should be glistening with sauce, and the mushrooms and shallots should be soft and caramelized.
   Tip For a complete meal, serve with a side of crusty bread to soak up the extra sauce.

After resting, the meat is moist and glistening. A generous scoop lifts tender beef strips, meaty mushroom slices, and sweet shallots coated in a velvety, golden-brown sauce. The sauce is deep and savory with a buttery finish.

Serve the steak and vegetables over a bed of creamy mashed potatoes, spooning extra sauce on top. The plate looks hearty and comforting, with steam rising and the rich aroma inviting you to dig in.

Common mistakes

The most common mistake is slicing the flank steak with the grain instead of against it. Slicing against the grain shortens the muscle fibers, making the meat more tender. Another mistake is overcooking on HIGH for too long, which can make the meat dry and stringy.

Stick to the recommended times and check for doneness with a thermometer. Also, avoid stirring the ingredients during cooking; the layers help the flavors develop and the sauce to form. Finally, do not skip the rest time—it is essential for juicy meat.

How to tell it’s done

The dish is done when the flank steak is fork-tender and easily pulls apart, and the mushrooms are soft and have absorbed the sauce. The internal temperature of the thickest piece of steak should reach at least 145°F (63°C) on an instant-read thermometer.

Because the meat is cooked in a moist environment, it will not dry out even if it cooks longer. Let the meat rest in the sauce for at least 3 minutes before serving to allow the juices to redistribute.

• Steaks, chops and roasts: 145°F (63°C) in the thickest part, then a 3-minute rest