Cowboy Butter Portobello Flank Steak: 5-Ingredient Crockpot Dinner Tips, FAQ & printable recipe

Tips for success

For the most tender flank steak, always slice against the grain—this cuts through the muscle fibers and makes each bite easier to chew. If the steak is very cold, it is easier to slice thinly; you can even pop it in the freezer for 15 minutes before slicing. Use a sharp chef's knife for clean, even strips.

Do not skip the resting time after cooking. Even though the meat is bathed in hot sauce, letting it rest allows the juices to redistribute through the beef, so it stays moist and flavorful. If you are not serving immediately, keep the slow cooker on the 'warm' setting for up to 2 hours, but check that the temperature remains above 140°F (60°C).

Variations

• Spicy Kick: Add 1-2 sliced jalapeños or a teaspoon of red pepper flakes along with the other ingredients. The heat will infuse the sauce and complement the buttery richness. For extra smokiness, use chipotle powder instead.
• Herb Swap: If you do not have cowboy butter, use a combination of butter, minced garlic, and dried herbs like thyme, rosemary, and oregano. You can also add a few fresh sprigs of rosemary to the slow cooker for a more pronounced herb flavor.
• Vegetable Boost: Add a cup of baby carrots or sliced celery along with the mushrooms and shallots. The vegetables will absorb the sauce and become tender. You can also add a handful of green beans during the last 30 minutes of cooking for a bright, crisp addition.
• Make It a Sandwich: Shred the cooked beef and pile it onto toasted hoagie rolls with provolone or mozzarella cheese. Broil until the cheese melts, then spoon extra sauce over the top for a delicious French dip-style sandwich.

Storage and make-ahead

Fridge

Store in an airtight container in the refrigerator for up to 4 days. Reheat gently with a splash of broth or water to loosen the sauce.

Freezer

Freeze in a freezer-safe container for up to 3 months. Thaw overnight in the refrigerator before reheating.

Reheat

Reheat in a saucepan over medium-low heat, stirring occasionally, until warmed through. Add a little beef broth if the sauce has thickened too much.

Store leftovers in an airtight container in the refrigerator for up to 4 days. The sauce will thicken as it cools, so add a splash of beef broth or water when reheating to loosen it.

Reheat gently on the stovetop or in the microwave, stirring occasionally, until heated through. The meat will remain tender and the flavors will continue to meld, making leftovers even more delicious.

This recipe is ideal for making ahead. Assemble the ingredients in the slow cooker insert the night before, cover, and refrigerate.

In the morning, place the insert in the slow cooker and cook as directed, adding 30 minutes to the cooking time to account for the chilled ingredients. You can also cook the dish fully, cool it quickly, and refrigerate for up to 3 days before reheating gently.

Serving ideas

The classic way to serve this dish is over creamy mashed potatoes, which soak up the rich, buttery sauce beautifully. The velvety texture of the potatoes pairs perfectly with the tender beef and meaty mushrooms.

You can also serve it over cooked egg noodles, rice, or polenta for a different twist. Add a side of crusty bread to mop up every last drop of the savory au jus.

For a lighter option, serve the steak and mushrooms over a bed of sautéed greens or roasted vegetables. The sauce is rich enough to stand on its own, so you do not need a heavy side.

Garnish with a sprinkle of fresh parsley or thyme leaves for a pop of color and a fresh herb note. This dish also makes excellent leftovers for sandwiches or tacos—just shred the beef and pile it high.

Frequently asked questions

Recipe card

Cowboy Butter Portobello Flank Steak: 5-Ingredient Crockpot Dinner

Prep 10 minCook 6 hRest 5 minTotal 6 h 15 minServes 6

Ingredients

• 2 lb flank steak, thinly sliced into strips
• 8 oz portobello mushrooms, thickly sliced
• 6 shallots, peeled and left whole
• 1/2 cup garlic-herb cowboy butter (2 sticks, cubed)
• 1 oz dry au jus mix

Instructions

1. 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.
2. 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.
3. 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.
4. 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.
5. 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.
6. 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.

Contains: milk

Estimated nutrition per serving

Calories

480

Protein

35 g

Carbohydrates

12 g

Fat

32 g

Automatic estimate from the ingredient list, not laboratory-verified.