Preheat the oven to 350°F. Spray a 2-quart casserole dish with nonstick cooking spray and set aside.

Heat the vegetable oil in a large skillet over medium heat. Add the chopped onions and cook, stirring occasionally with a wooden spoon, for about 5 minutes until softened and translucent.

Add the ground beef to the skillet. Cook for about 10 minutes, breaking it up with a wooden spoon or spatula, until browned and cooked through. Drain excess fat if needed.

In a large mixing bowl, combine the cooked beef and onion mixture with the baked beans, chopped tomatoes, brown sugar, Dijon mustard, salt, and black pepper. Stir with a large spoon until the mixture is well combined.

Pour the mixture into the prepared casserole dish and spread evenly. Lay the bacon slices across the top in a single layer.

Cover tightly with a lid or aluminum foil and bake for about 45 minutes, until thick and bubbly.

Remove from the oven and carefully remove the lid or foil. Preheat the broiler to high. Place the uncovered casserole under the broiler for 1–2 minutes, watching closely, until the bacon is crispy. Be careful not to burn it.

Let cool for a few minutes before serving.