In a small skillet, melt 1 tablespoon butter over medium-low heat. Add the mushrooms and saute until tender and slightly browned, about 5 minutes.
Spread butter evenly on one side of each bread slice and sprinkle evenly with garlic powder.
Layer one slice of cheese on the unbuttered side of 2 bread slices, then add the turkey, mushrooms, spinach, and bacon, dividing the ingredients evenly between both sandwiches.
Top each sandwich with the remaining cheese slices and the bread, leaving buttered side out.
Melt the remaining 1 tablespoon of butter in a large skillet and carefully transfer the sandwiches to the skillet.
Grill sandwiches on both sides until cheese is melted, spinach is slightly wilted, and bread is toasted to golden brown. Slice the sandwiches in half and serve immediately.