Mix the Filling: In a bowl, flake tuna with a fork. Stir in mayo, mustard, celery, onion, garlic powder, paprika, salt, and pepper.
Butter & Assemble: Butter one side of each bread slice. Flip slices over, then layer cheese on the unbuttered side of two slices. Pile tuna mix on top of the cheese, then top with remaining cheese and bread (buttered side out).
Crisp It Up: Heat a skillet over medium-low. Cook sandwiches 3–4 minutes per side, pressing gently with a spatula, until golden and cheese melts. For extra crunch, finish under the broiler for 1 minute.
Pro Tips
Bread Matters: Stale bread crisps better! No stale loaf? Lightly toast slices before assembling.
No Stick, No Problem: Use mayo instead of butter on the outside for a deeper golden crust.
Spice It Up: Add jalapeños, relish, or a dash of cayenne to the tuna mix.
Serving Suggestions
Serve with dill pickles, kettle chips, or a simple side salad. For a diner-style twist, cut diagonally (because triangles taste better, obviously).