Preheat oven to 325°F. Lightly coat a 9-inch pie pan with cooking spray.
Heat oil in a large skillet over medium heat. Add onion and cook, stirring, until softened, about 3 minutes. Add garlic and cook, stirring, until fragrant, about 1 minute. Stir in spinach; remove from heat.
Meanwhile, whisk milk, eggs, dill, lemon zest, salt and pepper together in a medium bowl.
Spread the spinach mixture evenly in the prepared pie plate. Sprinkle with feta, then pour in egg mixture. Bake until firm to the touch and a knife inserted into the center comes out dry, 45 to 50 minutes. Let stand for 10 minutes before serving. Garnish with additional fresh chopped dill, if desired.