Cowboy Butter Portobello Flank Steak: 5-Ingredient Crockpot Dinner

This slow-cooker cowboy butter portobello flank steak is the definition of set-it-and-forget-it cooking. Just layer thin strips of flank steak, meaty portobello mushrooms, and sweet shallots in the crockpot, top with garlic-herb cowboy butter and a packet of au jus mix, then walk away.

Hours later, you are rewarded with fork-tender beef and a rich, savory butter sauce that tastes like it simmered all day—because it did. With only five ingredients and almost no hands-on time, this is the kind of dinner that makes weeknights feel effortless.

This recipe takes a classic steak dinner and makes it completely hands-off. Flank steak is sliced thin and slow-cooked until it is meltingly tender, while portobello mushrooms soak up all the buttery, herby goodness. The shallots turn sweet and mellow as they cook, adding a gentle onion note to the sauce.

The cowboy butter—a mix of butter, garlic, and herbs—melts into the au jus to create a glossy, savory gravy that coats every bite. Serve this over creamy mashed potatoes or with crusty bread to soak up every last drop.

Ingredients

Why this recipe works

The slow cooker transforms a tough, lean cut like flank steak into fork-tender perfection. Low, steady heat breaks down the connective tissue in the meat, turning it silky and rich without any effort from you.

The long cooking time also allows the flavors of the butter, garlic, herbs, and au jus to meld and penetrate every strand of beef.

The combination of cowboy butter and au jus creates a deeply savory, umami-packed sauce with very little work. The butter adds richness and body, while the au jus brings a salty, beefy depth.

The mushrooms act as natural thickeners as they release their juices, and the shallots provide a subtle sweetness that balances the savory elements beautifully.

Notes and substitutions

Flank steak is the star here, and slicing it thin against the grain is key. This cut is lean and can be tough if cooked too quickly, but the slow cooker makes it wonderfully tender. If you cannot find flank steak, sirloin or skirt steak are good substitutes—just slice them the same way.

Portobello mushrooms bring a meaty texture that pairs perfectly with the beef. Thick slices hold up well during the long cook time and soak up the sauce without turning mushy. Shallots are sweeter and milder than onions, so they add a refined flavor; if you only have a yellow onion, cut it into wedges and use about half.

• flank steak: Sirloin steak or skirt steak, thinly sliced against the grain
• portobello mushrooms: Cremini or baby bella mushrooms, thickly sliced
• shallots: 1 small yellow onion, cut into wedges
• garlic-herb cowboy butter: 1/2 cup unsalted butter mixed with 2 minced garlic cloves and 1 tsp dried herbs (thyme, rosemary, or oregano)

Equipment

• 6-quart slow cooker
• Cutting board
• Chef's knife
• Measuring spoons
• Serving spoon
• Serving plate