Spread butter on one side of each of the slices of bread.
Spray cooking spray on skillet or grill pan and let preheat on low for a few minutes.
Place two pieces of bread onto skillet, buttered side down.
Top with slice of cheese, sliced avocado, slice of bacon, handful of spinach and then another piece of cheese.
Top with another slice of bread, buttered side up.
Press down with spatula and let cook for about 5 minutes.
Gently turn over and cook additional 5 minutes on the other side until golden brown.