Cook ground beef and onions: Preheat oven to 400°F. Heat oil in a large skillet over medium-high; add ground round and onion, and cook, stirring often, until beef is crumbled and no longer pink and onion is tender, 7 to 8 minutes. Drain all fat from skillet, and place over medium.

Add tomato paste, flour, and broth: Add tomato paste to beef mixture in skillet, and cook, stirring often, 1 minute. Sprinkle beef mixture with flour, and cook, stirring constantly, 1 minute. Gradually add broth, and cook, stirring constantly, until mixture thickens, 3 to 4 minutes.

Add tomatoes, seasonings, and cheese: Stir in diced tomatoes and chiles, mustard, Worcestershire sauce, salt, and pepper, and cook, stirring often, until mixture begins to simmer, about 2 minutes. Stir in ½ cup each of Monterey Jack cheese and Cheddar cheese, and cook, stirring often, until cheeses melt, 1 to 2 minutes.

Add casserole filling to baking dish: Spoon mixture into a lightly greased (with cooking spray) 11- x 7-inch baking dish.

Top with tots: Top evenly with tater tots. Place dish on a rimmed baking sheet.

Bake casserole: Bake in preheated oven, uncovered, until bubbly around edges and potatoes are golden brown, 30 to 35 minutes. Sprinkle top with remaining ½ cup each of Monterey Jack cheese and Cheddar cheese, and bake until cheese is melted, about 3 more minutes. Remove from oven, and let stand 10 minutes. Sprinkle casserole with crumbled bacon, and garnish with chopped chives. Drizzle with ketchup, if desired.