To make the mayonnaise, grill the garlic for 5 minutes until soft. Squeeze out the flesh and stir into the mayonnaise with the lemon zest and juice. Set aside.
Place the couscous in a bowl and pour over the boiling water or stock. Leave to soak for 15–20 minutes until the grains swell and all the liquid is absorbed.
Meanwhile, bone the sardines: place each one belly-down on a board and press down the backbone with your thumbs. Turn over and the bones should lift out easily. Snip the bone off at the tail end with scissors.
Stir the couscous with a fork to fluff up the grains. Stir the chopped tomatoes, capers, parsley and lemon zest into the couscous and season to taste.
Lay the sardines, skin-side down, on a board and spoon about 1 tablespoon of the couscous across the centre of each fish. Lift the wide end up over the stuffing, and bring the tail end over it. Secure with a wooden cocktail stick. Cook under a hot grill, turning once, for 6–8 minutes until golden and cooked. Add the vine tomatoes halfway through the cooking time. Spread the remaining couscous on a platter, arrange the fish and tomatoes on top and serve with the mayonnaise.