In a bowl, separate tuna into very small pieces. Add bread crumbs, onion, mustard, cayenne pepper, Italian seasoning and egg; mix well. In a medium saucepan, cook mixture for 2-3 minutes over medium heat. Set aside. Slice off 1/2 in. of each tomato top. Leaving a 1/4-in.-thick shell, scoop out pulp and discard. Spoon tuna mixture into shells and place in a baking dish. Top with mozzarella cheese and brown lightly under broiler. Garnish with sour cream if desired.