In a 12-inch cast iron skillet over medium heat, melt 2 Tbsp butter and cook the diced shallot until translucent.
Add the garlic to the skillet and sauté for about 30 seconds. Stir in the spinach and cook for an additional 1-2 minutes until wilted.
Remove the skillet from the heat. Spread the spinach evenly across the bottom of the pan. Add the roasted peppers and feta cheese.
In a large bowl, whisk together the eggs and heavy cream until well combined. Pour over the spinach mixture in the pan. Return the skillet to low heat and cook until the edges are set and beginning to brown.
Transfer the skillet to the oven and bake for 15-20 minutes or until the frittata is cooked through in the center and the knife comes out clean.