Stir together roasted red bell peppers, mayonnaise, red onion, parsley, 1 Tbsp. chopped fresh dill, Dijon mustard, pepperoncini salad peppers, liquid from pepperoncini salad peppers, and horseradish. Gently stir in Cheddar cheese and Havarti cheese until well blended. Cover and chill until ready to serve. Garnish, if desired.