Prep the tomatoes. Slice about 1/2 inch off the tops. Use a spoon to gently scoop out the seeds and pulp, creating a hollow.
Sprinkle the insides lightly with salt and place them upside down on a paper towel to drain while you make the filling.
Mix the dressing. In a large bowl, whisk together lemon juice, lemon zest, olive oil, garlic, and a pinch of salt and pepper. If you like heat, add the red pepper flakes.
Build the filling. Add the drained tuna, red onion, olives, capers, cucumber, parsley, and dill to the bowl. Fold gently to combine, breaking the tuna into bite-size flakes, not mush.
Add creaminess. Stir in the Greek yogurt or mayonnaise until the mixture is creamy but not wet.
Taste and adjust salt, pepper, and lemon juice. The flavors should be bright and savory.
Stuff the tomatoes. Pat the tomato interiors dry. Spoon the tuna mixture into each tomato, mounding it slightly on top.
Chill briefly. For the best texture, refrigerate stuffed tomatoes for 15–20 minutes to let flavors meld and the tomatoes firm up.
Garnish and serve. Finish with a drizzle of olive oil and a sprinkle of feta, chives, or extra dill.
Serve cool or at room temperature.