Preheat oven to 375°. Sauté spinach in oil and cook until excess water has evaporated. Add onion and scallions and cook on medium heat until onion is translucent. Add garlic and continue cooking for another minute. Remove from heat and cool slightly. In a mixing bowl add spinach mixture along with 1 cup of feta and dill. Season with salt and pepper to taste, keeping in mind the saltiness of feta cheese.

Clean mushroom caps and place on lightly oiled cookie sheet or baking dish. Fill each mushroom with 1 tablespoon spinach mixture and top with a bit of crumbled feta. Bake for 30 minutes until mushrooms are tender.