-Use frozen broccoli if that’s what you have, just be sure to thaw completely and pat dry before using. Add to skillet after cooking the onion and cook just to warm (don’t add water) before continuing with the recipe.
-Gruyére cheese would also be a nice substitute for the cheddar.
-The cooked frittata will keep for several days well wrapped in the fridge and can be rewarmed gently in the microwave. It does not freeze well.