Heat oven to 220C/200C fan/gas 7. Tip the sausages into a small roasting tin with the red onions and some seasoning. Drizzle with olive oil and roast for 10-15 mins, shaking halfway.
Meanwhile, whisk the plain flour with the eggs, milk, English mustard and some seasoning. Pour over the sausage-onion mixture. Scatter the gherkins and mini pickled onions into the mixture and bake for 25 mins.