Preheat the oven to 350°F.
Steam the broccoli florets until just tender, then set them aside to drain and cool slightly.
Chop the onion and add it to a frying pan coated with olive oil spray. Season with kosher salt and cook for 5 to 10 minutes, until tender and lightly caramelized.
Add the steamed broccoli to the onions and stir to combine. Set the vegetable mixture aside to cool briefly.
In a mixing bowl, combine the eggs, liquid egg whites, plain nonfat Greek yogurt, and cheddar cheese. Stir until evenly mixed.
Spray a baking dish with non-stick cooking spray.
Spread the cooled broccoli and onion mixture into the baking dish in an even layer.
Pour the egg mixture over the vegetables and gently shake the dish so the mixture settles evenly.
Bake for 30 to 35 minutes, or until the center is set with only a slight wobble.
Let the egg bake rest for 10 minutes before slicing.
Reheat portions in the microwave, air fryer, or toaster oven.