Instructions:
- Preheat oven to 375°F (190°C). Lightly grease a 9x13 inch baking dish if not using an oven-safe skillet.
- Heat olive oil in a large oven-safe skillet over medium-high heat. Brown the ground beef, breaking it up well. Drain off excess fat.
- Add the diced onion and mushrooms to the beef. Sauté until the onions are translucent and mushrooms have browned slightly (about 7-8 minutes). Stir in the minced garlic and thyme; cook for 1 minute until fragrant.
- Stir in the Worcestershire sauce, salt, and pepper. Remove skillet from heat. Gently fold in the 3 cups of cooked rice until evenly distributed.
- Make the sauce: In a separate saucepan, melt the 4 tbsp of butter over medium heat. Whisk in the flour and cook for 1 minute. Gradually whisk in the milk and beef broth until smooth. Bring to a gentle simmer, stirring constantly, until the sauce thickens enough to coat the back of a spoon (about 5-7 minutes).
- Remove the sauce from the heat. Stir in the Dijon mustard and the initial amounts of Cheddar and Jack cheese until completely melted and smooth. Taste and adjust seasoning.
- Combine & Transfer: Pour the cheese sauce over the beef and rice mixture. If necessary, transfer everything to the prepared baking dish. Stir gently until everything is coated.
- Top the casserole: Sprinkle the reserved 1/2 cup of shredded cheese evenly over the mixture. In a small bowl, toss the Panko breadcrumbs with 2 tbsp melted butter, then sprinkle this mixture on top of the cheese layer.
- Bake for 30-35 minutes, or until the filling is bubbly around the edges and the topping is deep golden brown and crisp. Let rest for 5-10 minutes before serving.