Grilled Apple, Bacon and Cheddar Sandwich with Roasted Red Onion Mayo

Sandwich

 * 2 slices (½-inch-thick) sourdough or another hearty bread
 * 4 slices thick-cut bacon, cooked crisp
 * 4 slices cheddar cheese
 * ½ Granny Smith apple, cored, sliced thin
 * Roasted red onion mayo, recipe follows
 * butter

Roasted red onion mayo

 * 1 medium red onion, chopped
 * 2 teaspoons olive oil
 * salt and pepper
 * 1 cup mayonnaise

Sandwich

 * 1) Preheat a seasoned grill pan or griddle over medium heat.
 * 2) Assemble this sandwich just as you would a traditional grilled cheese sandwich: spread 1 slice of bread with some red onion mayo, then top with 2 slices cheddar, the bacon, the apple slices, then 2 more slices of cheddar, finishing with another mayo-slathered slice of bread.
 * 3) Butter the outsides of the bread and transfer to the hot grill.
 * 4) Grill about 3 minutes per side, since this sandwich is so thick.
 * 5) Remove to a cutting board and cut the sandwich in half on the diagonal to serve.

Roasted red onion mayo

 * 1) Preheat oven to 375°F.
 * 2) On a small rimmed baking sheet, toss the red onion, olive oil, salt and pepper together.
 * 3) Roast in the oven for 25 to 30 minutes until the onion is very soft, stirring occasionally.
 * 4) Remove from oven and let cool.
 * 5) Transfer to a food processor and buzz until finely chopped.
 * 6) Add the mayo and pulse until smooth and combined.