Combine all the ingredients in a bowl and mix until well blended.
Transfer the mixture to a 4-cup baking dish and bake or smoke until the edges of the dip begin to crisp and brown, about 1 hour.
Recipe Tips
Blot juicy onions. Sweet onions hold a lot of water. If your diced onion looks wet, spread it on a paper towel for a few minutes before mixing so the dip does not turn watery.
Pick your dish for the crust you want. A wider, shallower dish gives more golden crust in every scoop, while a deeper dish keeps more of the dip soft and creamy. If you use a shallow dish, start checking a little early.
Let it rest before serving. The dip is molten straight from the oven. A few minutes of rest lets it thicken slightly so it clings to bread and chips instead of sliding off.
Taste the mixture before baking. Both cheeses are salty, so this recipe has no added salt. Adjust the cayenne and black pepper before the dip goes into the oven.