Place mushrooms gill side down and bake for 15 minutes.
While the mushrooms are baking, cook onions in butter until the onions are translucent.
Add spinach and spices to the onions and cook over medium flame until spinach is just wilted.
In a small bowl, mix milk and cornstarch together. Add to spinach and cook over medium flame until it the sauce begins to thicken.
Remove mushrooms from the oven. Flip them over and divide the spinach mixture between the mushrooms. Top with cheese. Bake for an additional 15 minutes.
Serve immediately.
Recipe by Premeditated Leftovers™ at https://premeditatedleftovers.com/?p=7067