Prep the produce: Clean and slice mushrooms. Thinly slice the onion. Mince the garlic. Grate or slice the Swiss cheese if not prepped.

Brown the beef: Heat a large skillet over medium-high. Add a splash of olive oil, then the ground beef. Season with salt and pepper. Cook, breaking into crumbles, until browned and cooked through, 5–7 minutes. Transfer beef to a plate and drain excess grease, leaving about 1 tablespoon in the pan.

Sauté the mushrooms: Add butter to the skillet. Stir in mushrooms and a pinch of salt. Cook undisturbed for 2–3 minutes to get color, then stir and continue until browned and their moisture evaporates, about 5–6 minutes.

Soften the onions: Add the sliced onion to the mushrooms with another small knob of butter if needed. Cook until soft and lightly golden, 4–5 minutes. Stir in garlic and cook 30 seconds, just until fragrant.

Build the sauce: Pour in beef broth, scraping up browned bits. Stir in heavy cream, Dijon mustard, Worcestershire, thyme, and a pinch of red pepper flakes if using. Simmer 2–3 minutes to thicken slightly. Taste and adjust salt and pepper.

Combine: Return the cooked beef and any juices to the skillet. Stir to coat in the creamy mushroom sauce. Simmer another 2–3 minutes until the sauce clings to the beef.

Add the cheese: Reduce heat to low. Lay Swiss slices over the top or sprinkle shredded Swiss evenly. Cover the skillet for 1–2 minutes until the cheese melts into the mixture.

Finish and serve: Sprinkle with extra thyme or cracked pepper. Serve as is, or spoon over steamed cauliflower rice or with a side of sautéed greens.