Warm the olive oil in a skillet over medium heat. Stir in the minced garlic and sauté for one minute, allowing it to become aromatic. Add the frozen spinach straight from the freezer, along with a pinch of salt and pepper, and continue cooking until the excess moisture has evaporated.

Generously butter one side of each slice of bread. Place them buttered-side down to begin assembling the sandwiches.

Layer half of the shredded mozzarella evenly onto two slices. Sprinkle crumbled feta on top of the mozzarella.

Spoon the cooked spinach mixture over the cheese layer, then add the remaining mozzarella and a light pinch of crushed red pepper flakes.

Complete the sandwiches with the remaining bread slices, buttered-side facing outwards.

Preheat a large skillet over medium-low heat. Once hot, place the sandwiches in the skillet and grill until each side is golden and crisp, and the cheese has melted to a gooey texture.

Slice each sandwich in half and serve warm.