Captain Butch Foster from yeah right charters (Southport North Carolina) has a bunch of recipes for spanish mackerals. heres one from his site. If you pm him im sure he'll give you his favorite. Theres a ton on his recipe page. i like the one cooked in wine.
SPANISH MACKEREL DELUXE 3 mackerel, 2 lb each; 2 tablespoons butter;1 tablespoon shallots, chopped; 1/2 pt wine, dry white;1/2 cup hollandaise sauce; 1 parsley sprig. Fillet the mackerel; place in a pan with butter and shallots, cover with wine and bring to a boil. Cook slowly for 12 minutes in hot oven. Remove the fillets, keeping them in a warm place. Reduce liquid in the pan by boiling. Add the juice of the parsley sprig which has been boiled and strained. Add 1 Tbsp butter and the hollandaise sauce, stir until smooth, pour over the hot fillets and serve. Can also be used for Kings and Amberjacks.