Preheat oven to 350 F. Lightly grease a 9" square baking dish.
To a medium skillet, saute the onion, garlic, and bell peppers until soft, about 5-8 minutes. Add the kale and allow to wilt.
Beat the eggs in a bowl, then add in the vegetables, cumin and cheese.
Transfer to the baking dish and bake for 25-30 minutes or until fully set. If you jiggle the pan it should just slightly wobble. Allow to cool.
Cut the frittata into 7 strips one way, and then once down the middle to yield 14 pieces. Then remove the pieces from the pan and cut them in half lengthwise to yield a total of 28 frittata fingers.
It's important to sauté your vegetables beforehand. You want to make sure the kale is nice and tender before adding them into the egg mixture and baking it. While you can add onions and bell peppers raw into the mixture, I find it more flavourful to sauté them first. Plus, it helps remove extra moisture.
Make sure you don't over bake your kale frittata. An over-baked frittata can turn out spongey so give it a check at the 25 minute mark before deciding if it needs to bake for the full 30 minutes.
It's best to season throughout (vegetables and eggs) and not skip step 3. You don't want to season the frittata mixture after you've put it into the pan since you want the seasoning to be throughout and not surface-level.