Holding it with tongs, char the pepper over a campfire, on a grill, or over a flame on a gas stovetop. When the outer skin is completely blackened, put the pepper in a paper bag and fold it over to close. Set aside for about 10 minutes.
Meanwhile, in a small bowl, combine the sliced serrano pepper and vinegar. Set aside.
Scrape the blackened skin off the pepper and remove the seeds and stem. Finely chop the remaining flesh.
Combine the sliced serrano, chopped pepper, cheeses, mayo, salt, pepper and 2 tablespoons of the beer in a bowl. Stir to combine. If it seems dry, add a tablespoon of beer at a time until it’s as moist as you want it to be. Serve with crackers, toasted bread or on a sandwich.