Heat oven to 375°F. In 2-quart saucepan over medium heat, melt 3 tablespoons of the butter. Add onion; cook and stir until softened, about 3 minutes. Stir in spinach, dill weed and flour. Cook an additional 5 minutes or until most of the liquid is absorbed. Remove from heat; stir in feta cheese, eggs, garlic powder, salt and pepper. Set aside.