In a large skillet, saute mushrooms in 1 tablespoon oil until lightly browned; remove with a slotted spoon and set aside. In the same skillet, saute spinach in remaining oil until wilted. Remove from the heat.
Spread bread slices with salad dressing and top with a cheese slice. Layer 4 slices with mushrooms, spinach and sauerkraut. Top with remaining bread. Butter outsides of sandwiches.
In a small skillet over medium heat, toast sandwiches for 2-3 minutes on each side or until cheese is melted.