Preheat the oven to 350°.
On medium-high, heat the cast iron skillet.
Once hot, add the crumbled bacon and cook until nearly done; about 3 minutes.
Add the onions to the bacon and continue cooking until they begin to soften; about 2 minutes
Drain any bacon renderings and place on medium-low.
Add the cream cheese, stir until melted, fully incorporating the bacon and onions.
Stir in the mayonnaise, mustard, garlic, salt, pepper to the creamy bacon and onions.
Stir in the roasted red peppers and 2 tsp. parsley.
Slowly mix in the cheddar cheese and ¾ of the pepper jack cheese.
Once the mixture has a melted and combined, sprinkle the remaining pepper jack cheese on the top.
Sprinkle the remaining parsley on top of the cheese.
Bake for 25 minutes.
Turn broiler on and place under boiler for about 2 minutes or until it's golden brown.
Remove from oven and serve immediately with crackers, baguettes, or chips.