Preheat your oven to 180°C (350°F). Lightly grease your baking dish to prevent sticking.
Heat a tablespoon of olive oil in a sauté pan over medium heat. Add the minced garlic and cook for about 30 seconds until fragrant, just starting to turn golden.
Add the chopped spinach to the pan and cook until wilted, about 2-3 minutes. Stir occasionally and remove from heat, letting the greens cool slightly.
In a large mixing bowl, whisk together the eggs, milk, nutmeg, salt, and pepper until the mixture is smooth and slightly frothy, about 30 seconds to 1 minute.
Gently fold the wilted spinach and garlic into the egg mixture. Then, add the shredded cheese and stir evenly to distribute.
Pour the mixture into your prepared baking dish, spreading it out evenly with a spatula to create a smooth surface.
Bake in the preheated oven for 20 to 25 minutes, until the top is golden brown and the edges are slightly crisp. The center should feel just set but still slightly jiggly when gently shaken.
Remove the baking dish from the oven and let it rest for about 5 minutes. This helps the casserole set and makes slicing easier.
Slice into squares or wedges, then serve warm, garnished with fresh herbs if desired. Enjoy the tender, custardy interior with a slightly crispy top.