Mix together the ale, vinegar, sugar, bay leaf, allspice berries, black peppercorns and thyme. Bring to the simmer and cook for a few minutes. Remove from the heat, cover with a lid and leave to cool. This will allow the spices to infuse the liquor. Lay the herring fillets, flesh-side down, in a lightly buttered, ovenproof dish, seasoned with a pinch of salt.
Re-boil the liquor and pour over the fish.
Place the dish on the stove and bring to the simmer. Remove from the stove and cover with a lid. Heat a frying pan with a knob of butter. Once bubbling, fry the sliced onions for 7-8 minutes, until softened and golden brown. Season with salt and pepper.