Pour 1 Tbsp olive oil into a large nonstick skillet over medium/high heat. Add potatoes, stirring often and cook until browned, 5 minutes. Add spinach and mushrooms and cook until potatoes and spinach is tender, additional 5-6 minutes.
While the potatoes are cooking, pour balsamic vinegar into a small saucepan over medium heat, and allow to boil lightly until consistency has thickened, roughly 1 minute. Wisk in raspberry preserves, 2 Tbsp olive oil, and mustard.
Place even amount of the mushroom, spinach and potato mixture onto salad plates, top with onion slices, crumbled Gorgonzola cheese and pistachios.
Drizzle balsamic dressing evenly over each serving.