Preheat the grill in the oven.
Pour boiling water over spinach in a bowl. Leave for 2 minutes or until the spinach just wilted, then drain. Rinse with cold water to cool, then drain.
Squeeze the water out as much as you can, then chop.
In a bowl, whisk together eggs, milk, and Parmesan cheese. Add the chopped peppers, and season with salt and pepper. Set aside.
Heat the oil in a pan and gently sauté onions until soft, for about 5 minutes. Add the garlic and cook a minute or so. Add the spinach and spread it out evenly.
Pour the egg mixture in the pan, let it spread out.
Scatter crumbles of the goat cheese over the top.
Put a lid on and cook for about 8 minutes on low heat.
When it's almost cooked through, put the pan in the oven and grill until the frittata is golden.
Remove from oven and let it cool for a few minutes.
Transfer it to a plate and cut it into quarters to serve.