Preheat the oven to 425°F (220°C). Lightly grease a large ovenproof baking dish of about 2.4 litre capacity, then sprinkle the breadcrumbs around the inside of the dish, including the sides.
Put the polenta in a large heatproof bowl and stir in the mustard, then pour in the boiling water and stir until smooth. Cool slightly, then stir in the egg yolks, corn, scallions, chives and about half of the cheese. Add the buttermilk and stir to form a thick batter.
Whisk the egg whites in a separate bowl until stiff peaks form. Beat 1 large spoonful of the egg whites into the batter to lighten it, then gently fold in the remaining egg whites.
Spoon the mixture into the prepared dish and sprinkle the remaining cheese over the top. Bake for 40-45 minutes until the top is set and golden brown. Serve at once, straight from the dish.
Tip: Buttermilk, a tangy cultured milk product, contains 2 grams of fat per cup, the same as a cup of 1% milk. It is stocked next to the cream in the cooler at supermarkets. If you can't find any, you can substitute 1% milk mixed with 1 teaspoon lemon juice and left to stand for about 5 minutes before using.
Variation: Boost the vegetable content by adding diced steamed carrots, sliced leeks or frozen peas (no need to thaw) with the corn in step 2.
Per serving: 492 calories, 13 g fat (5.5 g saturated), 19.5 g protein, 71 g carbohydrate, 5 g fibre