01 - Preheat oven to 375°F (190°C) and lightly grease a 6-cup muffin tin.
02 - Combine softened butter, minced garlic, and chopped parsley in a small bowl.
03 - Flatten each slice of bread with a rolling pin. Trim crusts as needed to fit muffin tin.
04 - Spread garlic butter on one side of each bread slice, then press bread butter-side up into each muffin tin cup, shaping a nest.
05 - Place a portion of grated Parmesan cheese in the bottom of each bread cup.
06 - Carefully crack one egg into each bread cup. Season with salt and black pepper. Sprinkle with mozzarella or cheddar cheese, if using.
07 - Bake for 16 to 18 minutes, or until egg whites are set and yolks reach desired doneness.
08 - Let cups cool for 2 minutes, then gently remove using a small spatula.
09 - Serve warm, garnished with additional parsley if desired.