Step 1

Preheat your oven to 400°F and grease a 9x13 inch baking dish. Heat a large skillet over medium-high heat and add the ground beef with a small dab of butter. Cook the beef, breaking it up into small crumbles as it browns - this takes about 6-8 minutes.

Step 2

Once the beef is cooked through, drain any excess grease. Add the diced onions to the pan and cook for another 3-4 minutes, stirring occasionally, until the onions start to soften and become fragrant.

Step 3

Remove the pan from heat and stir in the bacon bits, mayo, ketchup, mustard, salt, pepper, and onion powder. Mix everything together until well combined - this creates that classic cheeseburger flavor throughout the filling.

Step 4

Spread 6 slices of bread with half the margarine, then cut each slice in half. Place these pieces margarine-side down in your greased baking dish, creating the bottom layer. Sprinkle 2 cups of the cheese mixture over the bread.

Step 5

Spoon the beef mixture evenly over the cheese layer, spreading it out to cover the bread completely. Top with the remaining 2 cups of cheese - this creates those gooey, melty layers that make this casserole so amazing.

Step 6

Spread the remaining 6 slices of bread with the rest of the margarine and cut them in half. Place these pieces margarine-side up on top of the casserole. Bake for 20-24 minutes until the top is golden brown and crispy.

Step 7

Let the casserole rest for about 5 minutes before cutting - this helps it hold together better. Serve hot and watch everyone dig into this comfort food masterpiece!