Cut the mushrooms in halves (or quarters for large mushrooms). When the skillet is hot, add the mushrooms and cook for 5 minutes.
Add the garlic powder, black pepper, and cornstarch. Stir to mix. Cook for 30 seconds.
Add the milk, sun-dried tomatoes, and spinach and continue cooking over medium heat until the sauce is thickened and the mushrooms are as soft as desired, approximately 7 minutes.
Remove the skillet from the heat and stir in the Parmesan cheese until melted. Season to taste with salt and pepper.
Notes
I prefer to use mushrooms that are cut in halves or quarters instead of sliced because they keep their texture better. You can use any type or cut of mushroom as preferred.