Preheat oven to 400 degrees F.

Wash, dry, and cut stems off mushrooms. Dice up the stems. Brush mushroom caps with olive oil.

Melt butter in pan and add onions and mushroom stems. Saute until onion starts to become translucent. Next, add garlic and saute until garlic is cooked through. Finally, add spinach and saute briefly until wilted. Set aside to cool.

Once mixture is cooled, add the sausage, cornstarch, and egg. Spoon mixture into mushroom caps. Bake for 20 minutes until mushrooms are nicely browned.

Garnish with crumbled bacon before serving.