Make salsa, cutting tomatoes in thin wedges. Prepare a charcoal or gas grill for high heat (450° to 550°; you can hold your hand 5 in. above cooking grate only 2 to 4 seconds).
Meanwhile, rub halibut with olive oil and salt, pepper, and lemon zest to taste.
Grill halibut, turning often, until no longer translucent in center. Transfer to a platter.
Break fish into chunks, then tuck into tortillas, using 2 tortillas per taco. Spoon salsa over fish and tuck in cilantro sprigs.