Preheat oven to 375F and spray a standard size muffin pan with nonstick spray.

Preheat a skillet over medium-high heat and melt one tablespoon of butter. Add diced onions and sweet peppers and cook until soft, about 5 minutes.

Add chopped spinach and continue to cook until wilted. Remove from heat and pour vegetables onto a paper towel lined plate to absorb excess liquid.

In a large bowl, whisk eggs with heavy cream, salt and pepper.

Divide the sautéed vegetables among the 12 wells of your muffin pan. Crumble about a teaspoon of feta cheese on top of each.

Pour about ¼ cup of the egg mixture into each of the wells (about ¾ full). Top with more feta cheese and chopped chives, if desired.

Bake at 375°F for 20 to 25 minutes, or until the mini frittatas start to rise up and get puffy.

Remove from the oven and cool for 5 minutes in the pan. They will deflate as they cool.

Run a paring knife around the edges of each frittata to help remove them.

Serve immediately, or cool completely before storing. See storage instructions below.