Cook Ground Beef: Bring a large cast iron skillet to a medium-high heat. Add 1 pound ground beef to the hot pan and break apart. Season meat with kosher salt and ground black pepper and move around the pan to fully cook. Transfer to a bowl with a slotted spoon and drain excess grease.
Cook Veggies: Using the same skillet, add 1 tablespoon butter and return to a medium heat. Add onion and bell peppers to the skillet. Move around the pan and cook for 2-3 minutes or to your desired softness. Remove from the pan and set aside.
Make Cheese Sauce: In a separate saucepan, melt 2 tablespoons butter over medium high heat. Whisk in 3 tablespoons all-purpose flour and 1-2 teaspoons garlic to create a roux. Slowly add 1 1/2 cup half-&-half or creamer of choice, whisking continuously. Once it is all added, remove the saucepan from the heat and mix in the shredded cheese, ground mustard, salt and pepper.
Assemble Bowls: Evenly divide the beef, cooked veggies, and rice into individual bowls. Add your desired amount of cheese sauce on top and enjoy!
Storage: leftovers can be stored in an airtight container in the fridge for 3-4 days. Recommend storing the cheese sauce in a separate container to prevent sogginess and making it easier to reheat.