Heat the oven to 375°F and grease a 9-by-13-inch dish. Brown the beef and onion in a skillet and drain well.
Stir in the beans, tomato sauce, vinegar, brown sugar, mustard, Worcestershire, paprika and 1/2 teaspoon salt. Simmer for 5 minutes and spread in the dish.
Whisk the flour, cornmeal, baking powder and remaining 1/2 teaspoon salt. In another bowl whisk the egg, milk and melted butter, then stir the wet mixture into the dry just until combined.
Fold the cheddar into the batter and spoon it in small mounds over the beef, gently spreading to cover.
Bake for 30 to 35 minutes, until the cornbread is golden and a pick in its center comes out clean. Rest 10 minutes.