Combine the drained tuna, mayonnaise, Dijon mustard, red onion, parsley, lemon juice, salt, and pepper in a bowl. Stir until well-blended and creamy.

Take a slice of bread and generously spread a layer of the tuna mixture on it, reaching the edges for flavor.

Add fresh lettuce leaves and slices of cherry tomatoes on top of the tuna filling.

Place another slice of bread on top and gently press down.

Spread butter on the outside for a crispy finish before grilling or toasting if desired.

Continue the assembly process for remaining slices of bread until all sandwiches are made.

Slice the sandwiches in half or quarters for easy serving and arrange on a platter.