¼ cup each, finely diced onion, carrot and celery (from the outer stalks)
2 Tablespoons butter plus more for the baking dish
¼ teaspoon dried thyme or ½ teaspoon chopped fresh thyme
2 or 3 bunches celery
1 cup or more vegetable stock, chicken stock, or water
Salt and pepper
2 to 3 tablespoons butter, optional
Melt the butter in a small saucepan over medium heat. Add the diced vegetables, reduce the heat to slowly sweat the vegetables for about 5 minutes, stirring from time to time. Add the thyme and continue cooking for a few more minutes until the vegetables are tender. Add a little water if the vegetables start to brown. Set aside.
Remove the outer stalks of the celery, stopping when you get to the pale, tender ones. Cut off the tops of the hearts to make the hearts about 8 inches long. Carefully shave off any discolored parts of the root ends, leaving all the stalks attached.
Cut the hearts lengthwise into halves, thirds or quarters, depending on the size. Run cold water over the hearts to rinse.
Preheat oven to 350°F. Generously butter a flame-proof baking dish that will hold all the celery in one crowded layer. Spread half of the diced vegetables in the bottom of the dish. Arrange the celery cut side up over them, season with salt and pepper and spread the rest of the diced vegetables over the celery.
Heat the stock or water to a simmer in the small saucepan and pour enough into the baking dish to come 1/3 of the way up the celery. Lay over a piece of buttered waxed paper and cover with foil.
Bake for 40 minutes or more, until celery is very tender, but still holds its shape. Baste with the liquid in the dish from time to time.
Remove the celery to a serving dish and rapidly boil down the cooking juices until almost syrupy. Swirl in the optional butter and pour over the celery.
To serve cold, boil down the cooking juices, cool and make a vinaigrette sauce. Whisk in mustard, wine vinegar, minced shallots and olive oil to taste. Spoon over chilled celery and top with your choice of any or all of anchovies, chopped capers and parsley, chopped hard-cooked egg, or strips of roasted red pepper.