Preheat your oven to 375°F (190°C).

In a large mixing bowl, combine your ground beef with diced onion, chopped green bell pepper, minced garlic, breadcrumbs, milk, eggs, Worcestershire sauce, salt, and black pepper. Mix until well incorporated.

Grease a 9×9-inch baking dish and evenly spread the meat mixture into the bottom, pressing it down to create an even layer.

Sprinkle the frozen mixed vegetables evenly over the meat layer.

Spoon the mashed potatoes over the vegetable layer, smoothing it out with a spatula.

Spread ketchup over the mashed potatoes, followed by a sprinkle of shredded cheddar cheese.

Bake uncovered for 35–40 minutes, or until the cheese is bubbly and golden brown.

Let the casserole rest for 5-10 minutes before serving.