Spread mayonnaise on one side of each bread slice. Butter the opposite side of each slice.
On the mayo side of 4 bread slices, layer turkey, cheese, avocado slices, tomato, and spinach. Season with salt and pepper.
Top with remaining bread slices, mayo side down and buttered side up.
Heat a large skillet or griddle over medium heat. Place sandwiches buttered side down in the pan.
Cook for 3-4 minutes until golden brown and crispy, then carefully flip. Cook another 3-4 minutes until cheese melts.
Remove from heat, slice diagonally, and serve hot. Perfect with chips or a side salad!