Preheat the oven to fan 180C/ conventional 200C/gas 6. Pour the oil into the tin and put it in the oven for a couple of minutes to heat up. Add the sausages to the hot tin and roast for 10 minutes.
Push the sausages to the outer edges of the tin and scatter the sliced onion in the centre. Sprinkle the onion slices with the mustard seeds and some salt and pepper and turn them to coat in the hot oil at the bottom of the tin. Return to the oven for 10-15 minutes until the sliced onions are golden and the sausages are completely cooked through.
Briefly heat the flour tortillas in the oven, microwave or in a dry frying pan to make them softer and easier to roll. Place a sausage and some onion on each one, top with a spoonful of relish and roll, folding the bottom over. Serve straight away, wrapped in paper napkins.