Preheat oven to very hot. Grease deep 23cm-square cake pan, line base and two opposite sides with baking paper.
Place pumpkin in large microwave-safe bowl, cover; cook on HIGH,stirring halfway through cooking time, about 5 minutes, or until just tender. Place potato in small microwave-safe bowl, cover; cook on HIGH 4 minutes or until just tender.
Combine pumpkin and potato in large bowl; add spinach, cheeses and egg, stir to combine. Transfer egg mixture to prepared pan. Top with onion.
Bake in very hot oven about 25 minutes or until firm. Stand 5 minutes before serving.