Step 1 Preheat oven to 160°C. Line a 26cm x 16cm rectangular baking tin with baking paper.
Step 2 Spray a large frying pan with oil and place over medium-low heat. Add onion and cook, stirring occasionally, for 15 minutes, or until soft and golden. Add sugar and cook, stirring, for 2 minutes, until onion caramelises.
Step 3 Meanwhile, steam sweet potato for 8–10 minutes, until tender. Add zucchini in last 2 minutes of cooking. Add sweet potato, zucchini and spinach to onion and cook, stirring occasionally, for 2 minutes, until spinach wilts. Transfer to a bowl.
Step 4 Whisk eggs, egg whites and feta together. Add to potato mixture. Stir and season to taste.
Step 5 Pour mixture into prepared tin. Bake for 25 minutes, or until set, puffed and golden. Cool for 10 minutes. Remove from tin and cut into quarters. Serve with salad.