Drain tuna, chop onion and parsley. Shred cheese, set aside. In a large bowl,combine tuna, onion, parsley, Greek yogurt, mustard, lemon juice and salt and pepper.
Slice tomatoes into thirds (save the middle slices for later). Place tops and bottoms onto baking sheet, season to taste with salt and pepper.
Use Extra-Large Scoop to add tuna to bottom half of tomatoes. Top tuna with shredded cheese.
Broil 4-5 minutes, until cheese is melted and golden. Garnish tuna with with parsley and cover with top half of tomato, and serve immediately.
Notes
You can store any leftovers in a sealed container up to three days.