Preheat the oven to 200C/Gas 6/fan oven 180C. Tip the onions into a small shallow non-stick tin (about 23x30cm/ 9x12in). Arrange the sausages on top of the onions, then add the oil and roast for 20 minutes.
While they are roasting, make the batter. Sift the flour into a bowl, drop the egg in to the centre and beat in the milk a little at a time until it makes a smooth batter. Stir in the mustard and thyme and season.
Pour the batter quickly into the tin and return to the oven for 40 minutes until the batter is risen and golden. Serve with steamed carrots and cabbage.