Brown beef and onions with bouillon, salt, and pepper 8 to 10 minutes until fully cooked.
Add garlic. Cook 30 seconds. Drain excess fat.
Stir in tomato sauce, ketchup, mustard, brown sugar. Simmer 5 to 10 minutes until thick and glossy.
Spread mayonnaise on one side of each Texas toast slice.
Place bread mayo side down. Add cheese, ½ cup filling, more cheese, top bread mayo side up.
Grill 3 to 5 minutes per side until golden and melted. Adjust heat if needed.
Rest 1 minute. Slice and serve hot.