Pre-heat the oven to 200°C (400°F / Gas Mark 6). Put the onion wedges, oil and sausages into a small, shallow non-stick tin and roast for 20 minutes.
Prepare the batter by sifting the flour into a bowl, drop the eggs into the centre and beat these along with the milk a little at a time until it makes a smooth batter. Stir in the mustard, thyme and season to taste.
Pour the batter into the tin with the onions and sausages. Return the tin to the oven for 40 minutes until the batter has risen and turned golden.