In a small bowl, combine the softened butter, mayonnaise, minced garlic, fresh parsley, red pepper flakes (if using), kosher salt, and black pepper. Mix thoroughly until well combined and the garlic is evenly distributed throughout the spread. The mayo adds richness and helps emulsify the butter, creating a creamier spread that adheres better to the bread. Set aside while you prepare the remaining components.

Lay out all four bread slices on a cutting board. Spread the garlic butter mixture generously on one side of each slice—this side will face outward during cooking to create a crispy, flavorful crust. On two of the slices (butter-side up), distribute half of the shredded mozzarella and cheddar cheese blend evenly across each slice. I like to mix my cheeses right before using them so they don't clump together. Top each cheese-covered slice with one of the remaining bread slices, butter-side out, to create two sandwiches. Sprinkle the grated parmesan cheese on the outside of both sandwiches—this adds an extra crispy, savory crust.

Heat a skillet or griddle over medium-low heat for about 1 minute—medium-low is crucial here because it gives the cheese time to melt while the bread browns slowly and evenly. Once ready, carefully place both sandwiches in the pan. Cook for 4-5 minutes on the first side, pressing down gently with a spatula every minute or so to ensure even contact with the pan and help the cheese melt. You'll know it's ready to flip when the bottom is deep golden brown and crispy. Flip carefully and cook the second side for another 4-5 minutes until golden and the cheese is completely melted inside. I like to listen for the cheese to stop sizzling—that's when it's fully melted and the sandwich is done.

Transfer the grilled cheese sandwiches to a cutting board and let them rest for 30 seconds—this allows the cheese to set slightly so it won't ooze out when you cut. Slice each sandwich diagonally (this makes them look more appealing and easier to handle), and serve immediately while the cheese is still warm and gooey.