Preheat and prep: Heat your oven to 375°F (190°C). Lightly grease a 9x13-inch baking dish or an oven-safe skillet.

Cook the beef: In a large skillet over medium-high heat, add the ground beef and season with a pinch of salt and pepper. Cook, breaking it up, until browned with no pink remaining, about 6–8 minutes. Drain excess fat if needed, leaving about 1 tablespoon in the pan. Transfer the beef to a bowl and set aside.

Sauté the vegetables: In the same skillet, add butter or oil. Stir in the onions and mushrooms. Cook over medium heat, stirring occasionally, until the onions are soft and the mushrooms have released their moisture and browned slightly, 8–10 minutes. Add the garlic and cook for 30 seconds until fragrant.

Add seasonings: Stir in Worcestershire (if using), Dijon mustard, smoked paprika, thyme, 1 teaspoon salt, and 1/2 teaspoon pepper. Cook for 1 minute to bloom the flavors.

Make it creamy: Reduce heat to low. Add the cream cheese and stir until melted and smooth. Pour in the heavy cream and beef broth, stirring until the mixture is silky and slightly thickened, 2–3 minutes.

Combine with beef: Return the cooked beef and any juices to the skillet. Stir to coat everything in the creamy mushroom sauce. Taste and adjust seasoning.

Assemble: Transfer the mixture to your baking dish (or keep in an oven-safe skillet). Layer the Swiss cheese evenly over the top.

Bake: Place in the oven and bake for 12–15 minutes, until the cheese is melted and the edges are bubbling. For a golden top, broil for 1–2 minutes at the end, watching closely.

Rest and serve: Let the casserole rest for 5–10 minutes to set. Garnish with chopped parsley or chives. Slice and serve warm.