1. Preheat the oven to 180C/gas 4. Place the red pepper and red onion in a baking tray, drizzle on the rapeseed oil, sprinkle with black pepper and roast for about 15 minutes, until the vegetables start to brown. Remove from the oven and allow to cool.
2. Grease and line a 30cm x 20cm baking tin with parchment paper. For the base, pop the polenta, flours, baking powder, salt, black onion seeds and capers in a mixing bowl and stir. In a separate bowl, beat the eggs, milk and corn oil. Make a well in the centre of the dry ingredients, pour in the wet ingredients and stir with a spatula until well combined.
3. Spread the cooled onion and pepper mixture over the prepared baking tin. Spoon the batter on top and carefully spread to the edges. Bake in the oven for 20 minutes, or until golden brown.
4. Remove the tin from the oven and turn the oven to 200C/gas 6. Let the bumble-o cool slightly then spoon the onion marmalade on and spread it to the edges. Sprinkle over the grated cheeses and dried basil and bake the bumble-o for another 10–15 minutes or until golden brown and bubbling. Serve with green salad leaves.
Note: Sorghum flour is available from health food shops and Asian grocers and online.