Preheat oven to 400 degrees.
Remove stems from mushrooms, and set aside for later use.
Line a rimmed baking sheet with foil, and spray with nonstick cooking spray.
Place mushroom caps on tray, stem side down.
Brush tops of mushrooms with olive oil.
Bake for 8 minutes.
While mushrooms roast, finely chop mushroom stems.
In a nonstick pan over medium heat, add 1/2 tablespoon of olive oil.
When heated, add chopped steam, and sauté for 3 minutes, stirring occasionally.
Add garlic and sauté for one more minute, stirring frequently.
Remove mushrooms from the oven.
Reduce oven to 375 degrees.
Combine spinach, chopped mushrooms, garlic, goat cheese, salt, and pepper in a small bowl, and stir well.
Throw away foil, and line tray with a new sheet of foil.
Coat with nonstick spray.
Fill mushroom caps with spinach mixture.
Place filled caps on the prepared sheet, and bake for 8 minutes.
Serve.