Stir together first 3 ingredients plus salt and pepper to taste. Place a skillet over medium heat. Add 1 or 2 tortillas; sprinkle each with 1/4 cup cheese. When cheese melts, place 1/4 cup tuna salad and 1/4 cup spinach on 1 side of each tortilla; fold over. Press down with a spatula; heat 30 seconds. Repeat with remaining tortillas, cheese, tuna salad, and spinach.