In a small skillet, melt 1 tablespoon butter over medium-low heat. Add 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 ingredients evenly between both sandwiches. Top each sandwich with remaining cheese slices and the bread, leaving buttered side out.
Melt remaining 1 tablespoon butter in a large skillet and carefully transfer sandwiches to skillet. Grill sandwiches on both sides until cheese is melted, spinach is slightly wilted, and bread is toasted to golden-brown. Slice sandwich in half and serve immediately.