Pre-heat the oven to 200°C, then spray a roasting tin with low-calorie cooking spray.
Slice the sausages in half lengthways, and add to the roasting tin, along with the onion. Season to taste with salt and pepper. Cook for 20 minutes.
While the tray is in the oven, make the batter. Add the flour, milk, and eggs to a mixing bowl or jug, and whisk until smooth. Leave to one side while the sausages cook.
Remove the tray from the oven and quickly pour the batter around the sausages, then add the gherkins on top. Return to the oven for 20-25 minutes, until the batter has puffed up around the sausages (it should be golden brown and crispy).