Preheat oven to 350 degrees F (175 degrees C). Grease an 8-inch round pan.

Place sun-dried tomatoes into a bowl of warm water until rehydrated, about 10 minutes. Drain and chop.

Heat olive oil in a small skillet over medium heat; cook and stir onion and garlic until onion is translucent, about 10 minutes. Add spinach; cook and stir until thawed and water is evaporated, about 5 minutes. Stir in mushrooms, sun-dried tomatoes, and feta cheese until well mixed.

Whisk egg whites, skim milk, salt, pepper, and basil together in a bowl until very frothy. Carefully stir spinach mixture and 1 tablespoon Parmesan cheese into egg mixture. Pour into the prepared pan; top with remaining Parmesan cheese.

Bake in the preheated oven until frittata is set and browned on top, about 25 minutes.