Pre-heat oven to 350 degrees and place mushroom caps in a single layer on a baking sheet lined with foil.
Mince the reserved mushroom stems and heat olive oil in a large sauté pan over medium-high. Add grated garlic and minced mushroom stems to the pan along with salt and pepper. Saute for 2-3 minutes or until mushrooms are softened.
Add chopped spinach and chopped artichoke hearts to the pan and cook until heated through, about 3-4 minutes. Remove from heat and place mixture in a large mixing bowl.
Stir in feta cheese and minced scallions. Check for seasoning and adjust accordingly.
Using a small cookie scoop, add two tablespoons of the filling to each mushroom cap. Don't be afraid to fill it high.
Bake mushrooms for 20-25 minutes or until mushroom caps are softened. Sprinkle with chili flakes and minced scallions to serve.