Cowboy Butter Steak and Au Gratin Potato Casserole
Thinly sliced sirloin layered over boxed au gratin potatoes, soaked in a cowboy butter sauce — garlic, Dijon, Worcestershire, lemon, and red pepper flakes — and finished with melted provolone. True dump-and-bake comfort food that tastes like you did a lot more than you did.
Ingredients
Meat & Protein
- 1.25 lbs sirloin steak sliced as thin as possible against the grain — partially freeze for 20 minutes for easier slicing
Produce
- 5 garlic cloves minced
- 1 lemon juiced
- 3 tbsp fresh parsley roughly chopped, for garnish
Dairy
- 4 tbsp unsalted butter divided — 2 tbsp melted for cowboy butter sauce, 2 tbsp for the potato box preparation
- 1.5 cups provolone cheese freshly shredded or sliced thin
- 1.5 cups whole milk for the au gratin potato preparation
Pantry & Canned Goods
- 1 box au gratin potato mix 4.7 oz — Betty Crocker or Idahoan recommended
- 1 tbsp Dijon mustard
- 1 tbsp Worcestershire sauce
- 1 tbsp olive oil
Seasonings & Spices
- ¾ tsp kosher salt divided — ½ tsp for steak, ¼ tsp for cowboy butter sauce
- ½ tsp black pepper
- ½ tsp smoked paprika
- ½ tsp garlic powder
- ½ tsp crushed red pepper flakes adjust to heat preference
Instructions
- Preheat the oven to 400°F. Lightly grease a 9x13-inch baking dish with cooking spray or olive oil.
- Prepare the au gratin potatoes according to the box directions — combine the potato slices, sauce packet, milk, and butter directly in the prepared baking dish. Stir briefly to distribute the sauce packet evenly. Do not pre-cook.
- In a small bowl, whisk together the 2 tbsp melted butter, olive oil, minced garlic, Dijon mustard, Worcestershire sauce, lemon juice, smoked paprika, garlic powder, crushed red pepper flakes, ¼ tsp kosher salt, and black pepper. This is the cowboy butter sauce.
- Season the thinly sliced steak with ½ tsp kosher salt. Layer the steak slices evenly over the top of the potato mixture in the baking dish, slightly overlapping — they will shrink as they cook and nestle into the potatoes.
- Pour the cowboy butter sauce evenly over the steak slices. Use a spoon or brush to make sure the sauce covers the meat. Some will drip down into the potatoes — that is exactly what you want.
- Cover tightly with foil and bake for 40 minutes. The potatoes need to cook through and the sauce needs time to soak into everything before the cheese goes on.
- Remove the foil. The steak will be cooked through and the potatoes should be nearly tender. Scatter the shredded provolone evenly over the top. Return to the oven uncovered for 12 to 15 minutes, until the cheese is melted and beginning to brown at the edges and the potatoes are fully tender.
- Remove from the oven and let rest for 5 minutes. Garnish with fresh parsley and serve directly from the baking dish.
Notes
Thin slicing is the key to the steak cooking through evenly in the oven without a sear. Partially freeze the sirloin for 20 minutes before slicing — it firms up enough to slice paper thin with a sharp knife. The thinner the slice, the better it integrates with the potato layer underneath.
The cowboy butter sauce will sink down into the au gratin potato base as it bakes. This is intentional — the Dijon, Worcestershire, and garlic flavor the whole dish, not just the steak on top.
Provolone is the right cheese here — it melts cleanly and doesn't compete with the cowboy butter flavor. Sharp cheddar works as a substitute and will give a slightly sharper, more assertive finish.
Want more texture on the steak? See the blog post for the optional quick-sear method — sear the slices in a hot skillet for 60 seconds per side before layering. Adds a crust and links naturally to the Cowboy Butter Steak Bites method.
Leftovers keep for 3 days in the fridge. Reheat covered at 325°F for 15 minutes. The potato base tightens up as it sits — add a small splash of milk before reheating if it looks dry.
Tried this recipe?Let us know how it was!