Heat the olive oil in a 8 or 9-inch skillet, and add the onion. Cook over medium heat for 2-3 minutes, or until the onion softens. Add the kale, and cook uncovered for 2-3 minutes. Season with salt & pepper, stir one more time, and cover with the lid to let the heat inside the pan cook down the kale leaves for 1 minute.
Crack the 4 eggs to a bowl, season with salt and pepper, and beat the eggs with a fork or a whisk.
Lower the heat to the lowest it will go. Pour the eggs over the kale, and use a spatula to distribute the eggs evenly all around the pan. If there are any kale leaves sticking out out of the eggs, just push them into the egg mixture.
Arrange the tomato slices on top of the eggs. Sprinkle with 3-4 tablespoons shredded Parmesan cheese. Cover with the lid, and let it ''bake" inside the pan for about 5 minutes, or until the eggs are all set.
Remove from pan with a spatula, and slice into wedges.
You will need a skillet with a lid for this recipe. I recommend using an 8 or 9 inch skillet, otherwise the frittata will be too thin. This is a good one, with this universal lid.
15 Minute Vegetable-Loaded No Bake Frittata https://www.babaganosh.org/15-minute-vegetable-loaded-no-bake-frittata/