Preheat oven to 375° Fahrenheit. Lightly grease a cast iron skillet or pie dish.
In a skillet over medium high heat, add a splash of olive oil, the onions, broccoli leaves (or spinach or kale), and peppers. Saute for 3-5 minutes or until just golden and fragrant.
Whisk together the eggs, milk, cheese, and sauteed veggies until thoroughly mixed.
Pour egg mixture into greased skillet or dish. Bake in preheated oven for 20 minutes or until the frittata is golden around the edges and the middle is firm to touch.
Slice and serve warm. Leftovers can be stored in an airtight contatiner in the refrigerator for up to 3 days.