Get your fishmonger to gut and wash the Mackerel
Finely slice the Onion
Mix all the other ingredients up in a bowl – ie. the Rape Seed Oil, 2 tsp Tomato Puree, 1 tbsp Ketchup, 1 tbsp HP Sauce (or Wilkins BBQ sauce), very large dash Worcestershire Sauce, 2 tsp Cayenne Pepper, 2 tsp Smoked Sweet Paprika, 1 tsp Ground Coriander, 1 tbsp Sugar, 3 tsp English Mustard, 2 or 3 tbsp Vinegar (Red or White Wine)
Put the Mackerel into a roasting dish, and cover with the sliced Onions
Drizzle the marinade over the Mackerel (and rub into it, and inside the cavity of the fish)
Leave in the fridge to marinade for a few hours if possible
Pre-heat the oven to 180˚c
Roast the Mackerel for 20 – 30 minutes or until it is cooked (or starting to flake)
Serve with Sauté Potatoes and a light citrus-ey Salad