Combine the cheeses, mayonnaise, onion, and pepper, and stir until everything is thoroughly combined. Spread the mixture into small ramekins, small cast iron skillets, or a 6-inch baking dish. Lay the tomatoes on top. Bake for 40-60 minutes depending on the depth of the dip in your serving vessel(s) – you want the cheese bubbly, browning around the edges, and the tomatoes roasted.
Serve with gluten-free crackers, toasted gluten-free baguette slices, or vegetables.
Notes
To make this recipe dairy-free, swap out dairy-free cream cheese for the goat cheese and dairy-free Parmesan for the Manchego.