Clean mushrooms and cut into halves or quarters if large.
Heat 2 tablespoons of oil in a large skillet over high heat. Add mushrooms, sprinkle with salt and pepper, and cook, stirring occasionally (not constantly!), until they are golden brown and starting to release liquid, 3-4 minutes. Remove mushrooms from pan, place in a bowl, and set aside.
Add remaining 1 tablespoon oil to pan and reduce heat to medium. Add onion and garlic, and cook, stirring frequently, for 2-3 minutes or until onions are soft and translucent.
Add heavy cream, sun dried tomatoes, and oregano. Bring to a simmer and cook for 1-2 minutes or until starting to thicken.
Reduce heat to medium-low, add spinach, and cover. Cook with lid on for 2-3 minutes or until the spinach has wilted, stirring once or twice if needed.
Stir in cheese and mushrooms and cook for another 2-3 minutes or until the sauce has desired consistency.
Stir in basil and serve as desired. See notes for suggestions.
If using sun dried tomatoes that are not packed in oil, soak them in water before draining and patting dry. Refer to the package of sun dried tomatoes for directions.
Serving suggestions: Serve with pasta, rice, or bread, or as a side for steaks, chicken, or pork.