1. 1. Cut the leaves off the celery (you can reserve them for a soup), peel the stringy stalks and blanch them (cook an ingredient in simmering liquid) for 5 minutes in boiling salted water. Drain them.
2. 2. Grease the bottom of a sauté pan (a flat pan), place the celery in it, sprinkle with pepper and a little bit of stripped thyme, add 2 tablespoons of water, cover, and cook over low heat for 45 minutes to 1 hour.
3. 3. Meanwhile, peel and deseed the tomatoes (if they are fresh), cut them into pieces. Put them in a saucepan over low heat with 1 tablespoon of water, 1 sprig of thyme, bay leaf, 2 cloves of garlic, salt, and pepper. Cut and let cook for 10 minutes, then remove the thyme and bay leaf and blend the tomatoes.
4. 5. When the celery is tender, arrange them in a gratin dish, cover with the sauce, and bake for 15 minutes in a moderate oven to gratinate.