Line a 12-section muffin tray with paper muffin cases. Cut the pumpkin or squash into small dice, discarding the skin and seeds – you should have about 350 g (11½ oz) flesh. Spread out in a roasting tin with the onion slices and drizzle with 1 tablespoon of the oil. Roast in a preheated oven, 220°C (425°F), Gas Mark 7, for 30–40 minutes, turning once or twice, until beginning to colour. Leave to cool.
Mix together the flour, cornmeal, coriander, celery salt and baking powder in a bowl. Whisk together the eggs, milk and remaining oil with a fork in a jug.
Stir the cooled pumpkin or squash and onion into the dry ingredients. Add the egg mixture and mix together until the ingredients are only just combined. Divide the muffin mixture between the paper cases.
Bake in the oven for 20 minutes or until risen and just firm. Transfer to a wire rack to cool. Serve warm or cold.