Move a rack to the center of the oven and preheat the broiler to high heat. Line a sheet pan with parchment paper (safe for high temps) or foil and set aside.
In a small bowl, mix the butter, minced garlic, garlic powder, salt, parsley, mayo, and sour cream until smooth. Stir in both cheeses until well combined.
Slice the loaf in half horizontally and place it cut side up on the prepared pan. Evenly spread the cheese mixture over both halves.
Place under the broiler and cook until the cheese is melted and golden brown, about 3–5 minutes. Watch closely; it can burn in an instant!
Remove from the oven and let sit for about 5 minutes to cool slightly. Slice with a bread knife and serve warm.
Video
Notes
Storage: This cheesy bread is best enjoyed fresh! To freeze, spread the cheese mixture on the bread, wrap in foil, and freeze. Broil straight from frozen until hot and bubbly.If you won’t eat it all, cut the loaf into sections and broil only what you need, storing the rest of the butter mixture in the fridge. You can also use English muffins—just spread and broil on as many as desired.