Preheat oven to 400 F. In a mixing bowl, pour ¼ cup olive oil and 1 tbsp. balsamic vinegar, and sprinkle with garlic powder and sage spice. Gently coat mushrooms.
Mince 4 garlic cloves and set aside.
Remove the stems from 3 cups of fresh spinach and coarsely chop. Set aside.
Add 3 tbsp. olive oil to a frying pan and brown minced garlic for 3-4 minutes.
Place chopped spinach on top of the garlic and sauté for 4-5 minutes. As it wilts, gently stir into the garlic and oil. Sprinkle with salt and pepper and set aside when done.
Place mushroom caps face down in a pyrex or baking sheet and bake for 10 minutes.
Remove pyrex and pour out any liquid if any released. Gently turn caps over and stuff with spinach.
Sprinkle about 3 tbsp. of crumbled feta cheese (1½ tbsp. each) on top of the spinach. Bake for 20 minutes more and serve.