Grate the cheeses, slice/mince the onions, crumble (and cook if necessary) the bacon, drain and dry the pimentos, and drain, dry, and chop roasted red peppers.
Making the Dip
Preheat oven to 375°F.
Add the mayonnaise, softened cream cheese, salt, black pepper, ground mustard, granulated garlic, sweet paprika, and Worcestershire sauce to a large mixing bowl and cream everything together using an electric mixer until smooth.
Fold in the crumbled bacon, sliced green onions (reserving some green parts for garnish), diced pimentos, minced onion, and chopped roasted red peppers by hand using a rubber spatula or wooden spoon.
Stir in the shredded cheeses by the handful to the creamy pepper mixture. Do not add it all at once, as it could clump. Scraping down the sides while combining.
Add the cheese mixture to the cast iron skillet, garnish with reserved green onions, and bake for 15-20 minutes. For extra golden brown top, once baked, place under the broiler for a minute or two.
Serve and enjoy.
Notes
Shred your own cheese. Pre-shredded cheese has a cellulose coating that prevents smooth melting. A food processor makes quick work of it.
Soften the cream cheese. Room temperature cream cheese mixes much more evenly. Microwave for 15-20 seconds if you forgot to set it out.
Drain the peppers thoroughly. Excess liquid makes the dip watery. Pat both the pimentos and roasted reds dry with paper towels.
Do not skip the broiler. Two minutes under the broiler gives you a golden, bubbly top. Watch it closely.
Make ahead tip: Mix everything up to 24 hours in advance and refrigerate. Add a few extra minutes to the bake time if coming from the fridge.