Flake the tuna into a medium bowl. Mix in the mayonnaise, celery salt, onion powder and garlic powder. Stir in celery, onion and yellow capsicum.
Butter English muffin halves, place them on a baking tray and bake 3 minutes in the preheated oven, until lightly browned. Remove from oven. Spoon tuna mixture onto each muffin half and top each with a slice of cheese. Return to the oven and continue baking until the cheese has melted, about 8 minutes. Remove, and top each with a tomato slice to serve.