Prep the Bread: Start by spreading a thin layer of softened butter on one side of each slice of sourdough bread. This will give your melts a beautifully golden crust.
Mix the Cheese Filling: In a bowl, combine the shredded cheddar and mozzarella cheeses. Add garlic powder, salt, pepper, and red pepper flakes. Mix well to ensure that the spices are evenly distributed throughout the cheese.
Assemble the Melts: On the unbuttered side of two slices of bread, spread a generous layer of the mashed avocado and then top with a quarter of the cheese mixture. Add another slice of bread on top, buttered side facing out. Repeat with the remaining ingredients to make two more sandwiches.
Grill the Melts: Heat a large skillet over medium-low heat. Once hot, place the sandwiches in the skillet. Cook for about 3-4 minutes on each side, or until the bread is golden brown and the cheese is melted.
Serve and Garnish: Once cooked, remove the sandwiches from the skillet and let them cool for a minute. Cut them diagonally and garnish with fresh basil leaves.
Enjoy your Melts: Serve immediately while they’re warm and gooey.