Cut and discard stems of mushrooms. Spray each cap with cooking oil and place bottom side up and bake for about 2-3 minutes until natural juice forms in the center.
Meanwhile in a large saucepan, sauté garlic and onions until translucent. Add spinach ,salt and pepper. Sauté until spinach decreases in size. Add milk and cream cheese (optional) until mixture is slight creamy. Stir spinach filling and cook for additional 10-12 minutes
Heavily stuff the mushroom caps with the spinach filling and top with cheese. Bake for an additional 10 minutes until cheese is melted.
Garlic “au jus” sauce:
Strain the leftover “sauce” from the spinach mixture and discard any filling leftover. Use only the liquid as your au jus sauce.
When ready to serve, drizzle a little garlic au jus (filling liquid) on top (or on the side) and serve immediately.