Preheat a large griddle to 375°F. Meanwhile, gather all your ingredients—sourdough bread, bacon, turkey, cheese, tomato, lettuce, red onion, mayonnaise, and butter—so they’re ready to assemble quickly.
Take two slices of sourdough bread and open them like a book. Spread mayonnaise as desired on the inside faces of each slice, then close the bread back up. Next, butter the outside faces of the slices—these will touch the griddle and get golden and crisp.
Place the buttered bread, butter side down, onto the preheated griddle. On one slice, layer 1 slice of cheese, 3 strips of cooked bacon, 2 slices of turkey breast deli meat, 2 rounds of tomato, a few thin rings of red onion, and 1 leaf of lettuce for each sandwich. This hearty stack will create a delicious medley of flavor and texture—I always use freshly sliced tomatoes for maximum juiciness.
Let the sandwich cook on the griddle until both the bottom and top sides are golden and crispy, and the cheese is beginning to melt. Carefully monitor the heat to ensure the bread becomes perfectly toasted without burning. If you like your bread extra crispy, I suggest pressing the sandwich gently with a spatula while it cooks.
Once the sandwiches are nicely griddled, place the second slice of bread on top, cut each sandwich in half, and serve immediately. Enjoy your flavorful, melty creation!