Sauté the Spinach: Heat 1 tbsp olive oil in a pan over medium heat. Add garlic and cook for a few seconds. Add spinach, stir until wilted, then season with salt, black pepper, and red pepper flakes (if using). Let cool slightly.
Assemble the Quesadillas: Lay a tortilla flat and spread sautéed spinach over one half. Crumble block feta by hand on top, then fold in half.
Cook the Quesadillas: Heat a non-stick pan over medium heat. Brush with olive oil or butter and cook the quesadilla for 2-3 minutes per side until golden brown.
Serve: Cut into wedges and serve warm with tzatziki, yogurt, or a fresh salad.