Cut a thin slice off the top of each tomato and gently scoop out pulp and seeds, leaving a 1cm wall. Finely dice 1/2 cup of the pulp and set aside. Season the inside of each hollowed tomato with pinches of salt and pepper.
Mix reserved diced tomato pulp, goat cheese, mascarpone cheese, tuna, red onion, cucumber, mushrooms, red capsicum, celery, garlic and sesame seeds in a large bowl.
Scoop mixture into tomato shells up to the top of each tomato. Place on a foil-lined baking tray.
Sprinkle each tomato with sunflower seeds, Parmesan cheese, garlic salt, onion powder, salt and pepper. If desired, sprinkle each tomato with parsley flakes and Worcestershire sauce.
Bake in the preheated oven for 25 minutes, then grill right before serving until tops are lightly browned, about 1 to 2 minutes.