Preheat oven to 230 deg Celsius Line a large roasting pan with baking paper. Place pumpkin and onion in pan and lightly spray with cooking spray. Season to your liking with salt a pepper. Roast for 18-20 min or until vegetables are tender. Remove pan from oven place the spinach over the hot vegetables to wilt.
Line a 16.5 x 26 cm slab pan or baking disk with baking paper. Allow a 2 cm overhang at both long ends. Spread pumpkin, onion, and spinach over the base of the pan then top with feta.
Whisk eggs, eggwhites, milk, salt and pepper in a jug. Pour evenly over the feta and vegetables. Bake for 35 min or until set. Stand for 10 minute.