Prepare a charcoal grill for direct grilling, high heat.
Toss the tomatoes with oil and season with salt and pepper. Transfer to a grill basket and grill until charred and slightly soft, tossing occasionally, about 5 minutes
Combine the vinegar, sugar, coriander and cumin seeds in a small saucepan and bring to a boil. Reduce the heat and simmer until thickened to a glaze, about 10 minutes. Let cool slightly, then pour over the tomatoes. Toss, mashing the tomatoes slightly. Add the herbs and the zest, toss again, taste and season.