Preheat the oven to 425 °F & prepare all ingredients.
Using a large bowl, whisk together the eggs, milk, ¼ teaspoon of salt, cracked pepper, and ½ cup of cheese.
Heat a 10 inch cast iron skillet using avocado oil or other cooking oil (olive oil is not necessarily an ideal cooking oil). Add diced onions and stir until oil covers the onions. Stir frequently and cook until the onion is translucent and tender.
Add broccoli florets and water. Cover with a lid or sheet pan and cook for 2-3 minutes. The broccoli should be bright green and tender.
Uncover the skillet and add chopped spinach, garlic, green onion, and remaining salt. Stir until spinach has wilted. This does not take long.
Evenly spread and disperse the mixture across the base of the skillet.
Whisk the egg mixture again and pour over the greens mixture into the skillet.
Top the egg mixture with the remaining cheese.
Cook until you jiggle the pan and see that the middle of the egg is just barely set (12-15 minutes). When my frittata was done it had a nice big golden bubble on the top!
Let the frittata sit for 5-10 minutes. But don’t let it get cold! Frittata is best eaten right away, though it does keep quite swimmingly in the refrigerator!