Add the olive oil and minced garlic to a skillet. Sauté over medium heat for about one minute, then add the frozen spinach (no need to thaw), and a pinch of salt and pepper. Continue to sauté over medium until most of the moisture has evaporated from the skillet.

Spread butter over one side of each of the slices of bread. Flip the bread over so the buttered side is facing down as you build the sandwiches.

Divide half of the shredded mozzarella between two slices of bread. Top the mozzarella with the crumbled feta. Divide the cooked spinach between the two sandwiches, then top with the remaining shredded mozzarella and a pinch of crushed red pepper flakes.

Top each sandwich with the second slice of bread, buttered side facing out.

Heat a large skillet over medium-low. Once hot, add the sandwiches and cook on each side until golden brown and crispy on the outside and warm and gooey on the inside.

Cut the sandwiches in half and serve!