Heat a large non-stick pan over medium heat. Butter the outside of each slice of bread. On two slices of bread evenly sprinkle half of both cheeses on the inside of each slice. Top with gyro meat, onion, tomato, remaining cheese, and cover each sandwich with another slice of bread. Add sandwiches to pan and grill until golden brown and cheese has melted, about 2-4 minutes per side. Serve with homemade or store-bought tzatziki sauce on the side. Makes 4 half sandwiches.
Approximate Nutrient Analysis per serving (not including optional Tzatziki sauce):
400 calories, 20 g fat, 11 g saturated fat, 75mg cholesterol, 700 mg sodium, 31 g carbohydrate, 2 g fiber, 3 g sugar, 26 g protein