This recipe uses leftovers from the roast pork with fennel, chiles, and olives, so be sure to save both recipe cards.
Preheat oven to 375 degrees. Cut off and discard top 1/2 inch of garlic head. Season garlic with salt, drizzle with oil, and wrap snugly in foil. Roast until tender when squeezed, 50 to 55 minutes.
Meanwhile, toss onion with just enough oil to coat, and roast on a rimmed baking sheet 40 to 45 minutes.
When garlic is cool enough to handle, squeeze from skin; lightly mash with a fork. Stir in mustard and mayonnaise.
Raise heat to broil. Spread garlic mayonnaise on bottom halves of rolls; top with pork. Top remaining halves with vegetables, then cheese. Broil open-faced until cheese is melted and bubbling, about 2 minutes, then sandwich rolls together.