Preheat the oven to 400°F. Spray a 9x13-inch baking dish with nonstick cooking spray.
In a large skillet, over medium-high heat, add the olive oil and ground beef. Cook and crumble the ground beef until it's no longer pink.
1 tablespoon olive oil, 1 pound lean ground beef
Add the finely diced onion and green bell pepper into the ground beef. Continue cooking, while stirring often, until the ground beef is browned and the vegetables are softened. Drain any excess grease from the skillet if needed.
1 yellow onion, 1 green bell pepper
Stir in the Worcestershire sauce and beef broth, stirring well and scraping any browned bits from the bottom of the skillet.
3/4 cup beef broth, 2 teaspoons Worcestershire sauce
Add the garlic powder, kosher salt, and black pepper. Cook for about 30 seconds until fragrant.
2 teaspoons garlic powder, 1 teaspoon kosher salt, ¼ teaspoon black pepper
In a small bowl, whisk together the cold water and cornstarch. Pour the mixture into the skillet and simmer everything together for 3-5 minutes, stirring often, until the beef mixture thickens and looks glossy and saucy.
2 tablespoons cold water, 1 tablespoon cornstarch
Spread the ground beef mixture into the prepared pan in an even layer. Arrange the frozen tater tots in a single, even layer on top of the beef and then sprinkle the shredded mozzarella cheese evenly over the tots.*If you don't want the cheese to be golden brown, and prefer the tots crispier and browned, then layer the cheese on top of the ground beef layer and then add the tater tots on top (this is usually how I do it). 1 bag (32 oz) frozen tater tots, 2 cups shredded mozzarella cheese
Bake uncovered for 35-40 minutes until the tater tots are browned and the cheese is melted and bubbling.
Let the casserole rest for about 5 minutes so it sets slightly, then scoop hot, and enjoy!