In a small skillet, heat olive oil over medium heat and sauté the minced garlic for 30 seconds until fragrant.

Add the fresh spinach to the skillet and cook until wilted, approximately 2 minutes.

Season the spinach with salt, black pepper, and red pepper flakes, then remove from heat and drain any excess liquid.

Spread butter evenly on one side of each bread slice.

Place two slices of bread, butter-side down, in a clean skillet or griddle over medium heat.

Layer each slice with a portion of mozzarella, the sautéed spinach, and crumbled feta.

Top with the remaining mozzarella and the other bread slices, butter-side up.

Grill for 3 to 4 minutes per side until the bread is golden brown and the cheeses are fully melted.

Remove from heat, let rest for 1 minute, and slice diagonally before serving.