Grate the zucchini and sprinkle with the salt. Set it aside for about 15 min to pull the water out.
Peel the sweet potato and shave it into ribbons with a Y-peeler. Slice the onion into thin rings.
Spread the sweet potato ribbons and onion rings on a parchment tray with a light spray of avocado oil and a pinch of salt. Roast at 400°F for 12-15 min until tender.
While that roasts, whisk the egg whites, eggs, Greek yogurt, Parm, Dijon, garlic powder and pepper until smooth.
When the sweet potato and onion come out, drop the oven to 350°F. Squeeze all the water out of the zucchini in a towel, then fold the zucchini, roasted sweet potato and onion into the egg mixture (save a few ribbons to decorate the top).
Pour into a parchment-lined 8x8 pan and arrange the reserved ribbons on top.
Bake at 350°F for 50-60 min until set, a knife comes out clean and the top's a nice golden brown.
Rest 10-15 min before slicing into 4. Store in the fridge up to 4 days, or freeze for longer. Reheat in the microwave or air fryer.
