Preheat the oven to 350°F (175°C).

In a large bowl, whisk together the eggs and milk. Season with salt and black pepper.

Heat a non-stick skillet over medium heat. Add the spinach and cook until wilted, about 2-3 minutes.

Add the diced red onion and cook for another 2 minutes.

Pour the egg mixture into the skillet and top with crumbled feta cheese and chopped dill.

Transfer the skillet to the preheated oven and bake for 15-20 minutes, or until the frittata is set and golden on top.

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

Enjoy!