Preheat the oven to a temperature of 400°F (200°C).

In a skillet of ample size, placed over medium heat, brown the lean ground beef. Once the beef has browned, incorporate the finely diced onion and green bell pepper. Continue to cook until the vegetables have softened, which should take approximately 5 minutes.

To the skillet, add the brown sugar, Worcestershire sauce, dry mustard, and garlic salt. Mix thoroughly. Subsequently, incorporate the tomato sauce and ketchup, stirring until the ingredients are uniformly combined. Allow this mixture to gently simmer on low heat as you proceed with the next steps.

For the casserole topping: In a separate mixing bowl, combine the all-purpose flour, baking powder, and salt. Slowly incorporate the milk, followed by the melted butter and egg. Mix until these ingredients are uniformly combined. Finish by gently folding in the shredded cheddar cheese.

Transfer the meat mixture from the skillet to a baking dish measuring 13 by 9 inches. Carefully distribute the prepared topping over the meat mixture, ensuring an even coverage.

Place the dish in the preheated oven and bake for a duration of 25 to 30 minutes, or until the topping achieves a light golden brown hue. Remove from the oven and allow it to cool slightly before serving.