Preheat the oven to 220C/425F/Gas Mark 7. Divide the oil, SPAM® and onion in a roasting tin. Place the tin on a baking sheet and bake for 10 minutes.
Meanwhile, place the remaining ingredients together in a large jug with 150ml/1/4pt water and a good pinch of salt. Beat together until smooth, then set aside.
Remove the tin from the oven, pour in the batter, and then quickly return it to the oven and bake for 30-35 minutes or until golden and risen. Serve immediately with onion gravy.