Heat the grill on the hottest setting. Rinse the sardines and pat dry with kitchen paper. Make 3 deep slashes on both sides of each fish with a sharp knife.
Mix the harissa with the oil and lemon juice to make a thin paste. Rub into the sardines on both sides. Put the sardines on a lightly oiled baking sheet. Cook under the grill for 3–4 minutes on each side, depending on their size, or until cooked through. Season to taste with salt flakes and pepper and serve immediately garnished with coriander and with lemon wedges for squeezing over.