Cook beef in a large skillet over medium-high heat, breaking up any large chunks with the back of a wooden spoon, until no longer pink, about 4 minutes.

Add onion, red bell pepper, and garlic, and cook, stirring occasionally, for 1 minute. Remove the pan from the heat and stir in Worcestershire and ¼ teaspoon salt and ½ teaspoon black pepper. Set aside.

Heat butter in a large pot over medium heat. Add mushrooms and cook, stirring occasionally, until browned and no liquid remains, about 4 minutes.

Add green beans and shallots, and cook, stirring occasionally, until the vegetables are tender, about 3 minutes.

Sprinkle the vegetables with flour and thyme. Cook, stirring, for 30 seconds.

Slowly pour in broth and milk. Cook, stirring, until the sauce is thickened, about 3 minutes. Season with ¼ teaspoon salt and ¼ teaspoon pepper.

Evenly divide the ground beef between two 1-cup and/or 2-cup Souper Cubes tray, or a 9x13 inch baking dish.

Top the beef with the mushroom sauce. Sprinkle the cheese over the sauce and then top with the tater tots.

If freezing, let the casserole cool completely before covering with the Souper Cubes lid. Freeze until solid, at least 24 hours.

If eating immediately, bake in a 350°F degree oven until bubbling around the edges and lightly browned on top, about 45 minutes.

Preheat oven to 400°F.

Place a frozen cube in a baking dish. Cover with foil and bake until no longer frozen in the center, about 45 minutes for a 1-cup portion and 55 minutes for a 2-cup portion. Remove the foil and continue baking until bubbling throughout and lightly browned on top, 10 to 15 minutes more.