The Vietnamese love their salted dishes, and salty seafood is no exception. In many recipes, seasonings like salt are often taken for granted. We do not have that situation here. We use an appetizing salt rub that permeates the skin of the fish and makes it nice and crispy. Mackerel has a high oil content, which makes it a perfect candidate for grilling. If you prefer another fish, ask your fishmonger for the freshest oily white fish available. For this recipe, and for grilled fish in general, you really need to use a whole fish, preferably not deboned, as fillets tend to flake and fall apart while grilling. If you feel you must debone your whole fish before cooking it, stuff it with scallions and tie it up with cooking twine so that it maintains its shape. The skin needs to stay on, as this is what keeps the flesh intact while grilling.