Making a Spinach and Feta Frittata is straightforward if you follow these easy steps:

1. Preheat the Oven: Start by preheating your oven to 375°F (190°C).
2. Sauté the Onions: Heat olive oil in an oven-safe skillet over medium heat. Add chopped onion and sauté until translucent, about 2-3 minutes.
3. Add the Spinach: Stir in the chopped spinach and cook until wilted, about 2-3 more minutes. Remove from heat.
4. Prepare Egg Mixture: In a mixing bowl, whisk together the eggs, milk, salt, pepper, and garlic powder until well combined.
5. Combine Ingredients: Add the sautéed spinach and onion mix into the egg mixture along with crumbled feta cheese. Stir to combine.
6. Pour Mixture into Skillet: Wipe the skillet if necessary, then pour the egg and vegetable mixture back into the skillet.
7. Cook on Stovetop: Cook on the stovetop for about 2-3 minutes until the edges start to set, gently lifting the sides with a spatula to let uncooked egg flow underneath.
8. Bake: Transfer the skillet to the preheated oven and bake for about 15-20 minutes, or until the frittata is set in the center and lightly golden on top.
9. Cool and Slice: Remove from the oven and allow to cool slightly. Slice into wedges and serve warm or at room temperature.

With this simple guide, you will have a delicious and satisfying Spinach and Feta Frittata in no time!