1. Preheat the oven to 190°C.

2. In a medium bowl, whisk together the eggs, heavy cream, salt, and black pepper until well combined.

3. Heat the olive oil in a large oven-safe skillet over medium heat. Add the onion and garlic and cook until softened, about 3 minutes.

4. Add the chopped spinach to the skillet and cook until wilted, about 2 minutes.

5. Pour the egg mixture over the spinach and onion mixture in the skillet.

6. Sprinkle the crumbled feta cheese over the top of the egg mixture.

7. Place the skillet in the preheated oven and bake for 15-20 minutes or until the frittata is set and golden brown on top.

8. Remove the skillet from the oven and let it cool for a few minutes before slicing and serving.

Enjoy your delicious keto-friendly Spinach and Feta Cheese Frittata!