Stovetop only — no oven needed; cook over medium-low so the cheese melts before the bread burns. Drain the tuna very well (press it in a fine-mesh strainer) to avoid a soggy sandwich. Cheddar can be swapped for pepper jack, Swiss, or American; tomato can be omitted. Tuna salad can be made up to 5 days ahead and refrigerated in an airtight container.
