In a cast iron skillet (or other ovenproof skillet) over medium heat, melt the butter. A

dd in the onion and garlic, and sauté for five minutes or until the onions have softened and are translucent.

Meanwhile, beat the eggs in a bowl and season with salt and pepper.

Add the fresh or frozen spinach to the skillet; allow the fresh spinach to wilt, or let the thawed spinach cook away some of its water content.

Add the eggs, turn the heat to low and stir gently until the eggs begin to thicken a bit.

Add in the herbs and stir to incorporate.

Once the eggs have set, sprinkle the parmesan on top (optional), then transfer the skillet to the broiler.

This will cook the top of the frittata and brown the cheese, if using.

Check after two minutes in the broiler.

Allow to cool for ten minutes, then serve warm. This frittata is also delicious served cold.