Heat broiler. In a large bowl, combine tuna, onion, parsley, mayo, mustard, and lemon juice and season with salt and pepper.
Slice tomatoes into thirds. (Save the middle slices for later.) Place tops and bottoms face side up on a baking sheet. Spread tuna mixture on bottom halves and top with cheddar. Season top halves with season and pepper.
Broil until cheddar is melty and golden, 4 to 5 minutes.
Recipe by The Best Video Recipes for All at https://recipes-for-all.com/tomato-bun-tuna-melts/