Preheat oven to 375.
Chop the onion; you should have about 2 cups.
Place the chopped onions and the cheese and mayo in a large mixing bowl and stir to combine well. Transfer the mixture to a 2-quart glass baking dish and sprinkle the top with the cayenne.
Bake until the dip is golden brown and bubbling throughout, 22 to 25 minutes.
Serve the dip warm, right from teh casserole, with chips.