Preheat oven to 375°F (190°C). Grease a 9-inch pie dish or baking dish.

In a large bowl, whisk together eggs, milk, cheddar cheese, Parmesan cheese, salt, and pepper.

In a skillet over medium heat, warm 1 tbsp olive oil. Add 1 small yellow onion and cook until soft (4-5 mins). Add 2 cloves garlic (1 min).

Stir in 10 oz. fresh spinach and sauté until wilted (2-3 mins). Remove from heat, let cool slightly, drain thoroughly, pressing out excess liquid until no more than 1 tbsp of liquid remains. Add to egg mixture.

Pour into the prepared dish. Bake for 30-35 minutes, or until the center is set and the top is lightly golden.

Cool for 5-10 minutes before slicing. Serve warm or at room temperature.