Wrap the garlic, ginger, rosemary and thyme in cheesecloth and tie in a bundle. Heat the olive oil in a large casserole. Add the onion and cook over moderately low heat until softened, about 10 minutes. Add the tomatoes and herb bundle, season with salt and pepper and cook over low heat until the tomato juices have reduced by one-fourth, about 20 minutes.
Mix the cornstarch with the water, add to the tomatoes and simmer until slightly thickened, about 8 minutes. Add the celery seeds and season with salt and pepper. Remove the herb bundle and serve.