Drain the tuna hard: Empty the cans into a fine-mesh strainer set over the sink and press down firmly with the back of a spoon until the liquid stops running. This is the single biggest thing standing between you and soggy bread.

Mix the tuna salad: Flake the drained tuna into a bowl and stir in the mayonnaise, celery, red onion, relish, Dijon, lemon juice, salt, and pepper. Taste and adjust. The salad should hold together in a scoop, not slump.

Butter the bread: Spread the softened butter on one side of all 8 slices of bread. Softened is important; cold butter tears the bread instead of spreading across it.

Build the sandwiches: Turn 4 slices butter-side down on a cutting board. Lay a slice of cheddar on each, then divide the tuna salad over the cheese and spread it to the edges. Add tomato slices if using, then a second slice of cheddar on top. Close with the remaining bread, butter-side up.

Griddle the tuna melts: Heat a large skillet or griddle over medium-low. Cook the sandwiches for 3-4 minutes per side, pressing lightly with a spatula, until the bread is deep golden and the cheese is fully melted. Cook them in two batches if your pan is crowded.

Watch the heat: If the bread is browning before the cheese melts, drop the heat to low and give it another minute or two. Medium-low is what lets the inside catch up to the outside.

Rest and slice: Let each sandwich sit for 1 minute on the cutting board so the cheese sets slightly, then slice on the diagonal and serve hot.