Preheat your oven to 180°C (350°F). Grease a baking dish with a little oil or butter and set aside.
Heat a skillet over medium heat, add a teaspoon of olive oil, and sauté chopped onion until translucent, about 3 minutes. Add minced garlic and cook for just 30 seconds until fragrant.
Add the chopped spinach to the skillet. Cook, stirring, until wilted and bright green, about 2-3 minutes. Remove from heat and let cool slightly.
In a mixing bowl, whisk the eggs until smooth and slightly frothy. Stir in milk or cream if using, and season with a pinch of salt and pepper.
Fold the grated or crumbled cheese into the egg mixture, distributing evenly.
Add the sautéed spinach mixture to the eggs and gently fold everything together until well combined.
Pour the combined mixture into the prepared baking dish, spreading it out evenly with a spatula.
Bake in the preheated oven for 25-30 minutes, or until the top is golden brown and the edges are slightly crisp. The center should be just set but still a little jiggly.
Remove from the oven and let rest for 5 minutes; this helps it set and makes slicing easier.
Slice into squares or wedges, serve warm, and enjoy the comforting, fluffy texture with vibrant greens and melted cheese.
