Preheat the oven to 375 and grease or spray a muffin tin.
Dice your onion and radishes.
Heat the olive oil in a medium-sized skillet over a medium-high heat. When the oil is shimmering, but not smoking, add the onion and radishes and sauté, stirring occasionally, until the onions and radishes are starting to brown, around 5 minutes.
While the onions and radishes are sautéing, wash and roughly chop your arugula.
Crack the eggs into a large bowl and whisk until well combined. Stir in the feta, milk, basil, salt, and pepper.
When the onions and radishes are ready, add them to the egg mixture, along with the arugula, and mix well.
Divide the mixture up evenly in the muffin tin, a little less than ¼ cup per cup.
Bake for 20-22 minutes, or until they are slightly firm to the touch. Remove from the oven and let them cool for 5 minutes. Run a small knife around the outside of each frittata and remove gently to cool on a plate or cooling rack or serve right away.
Recipe by The Balanced Kitchen at http://www.balancedkitchen.com/2017/06/arugula-and-radish-mini-frittatas/