Preheat your oven to 375°F (190°C).
In a large skillet, heat olive oil over medium heat. Add diced onion and sauté until translucent, about 5 minutes.
Add minced garlic and chopped spinach to the skillet. Sauté until the spinach wilts, about 2-3 minutes.
In a bowl, whisk together eggs, ricotta, parmesan, salt, and pepper.
Pour the egg mixture into the skillet over the sautéed vegetables.
Cook on the stovetop for about 2 minutes until the edges start to set.
Transfer the skillet to the preheated oven. Bake for 18-20 minutes, or until the center is set and the top is lightly golden.
Remove from the oven and let it cool for a few minutes before slicing.
Serve warm or at room temperature, garnished with additional cheese or herbs if desired.