Preheat the oven to 400 degrees. Lightly film the bottom of a 12-inch straight-sided saute pan with oil. Heat over medium and add the bacon. Cook slowly until the bacon is clear and has given up much of its fat. Don't let it get beyond pale golden brown. Scoop it up with a slotted spoon and set aside.
Spoon off all but about 5 tablespoons of fat from the pan. Heat over medium high and add the celery hearts, sprinkling them with salt and pepper. Brown them on all their sides, but take care not to let the glaze developing on the bottom of the pan burn. Lift them out and add the carrots to the pan, brown and then add the onion, cooking until it softens. Don't let the glaze burn.
Put the bacon and celery back in, along with the lemon zest, fennel, thyme or oregano, and water. Simmer as you use a wood spatula to scrape up the glaze from the bottom of the pan. Blend in the tomatoes. Moisten the vegetables with a little more water and cover the pan with a lid or foil to seal it.
Slip into the oven and cook 35 to 40 minutes, or until vegetables are tender. Check halfway through cooking to make sure they're not burning, adding a few spoons of water if needed.
Once the vegetables are tender, carefully remove the pan from the oven (the handle will be hot). Mound the vegetables on a platter and scrape the syrupy pan juices over them. Serve hot. Rice or buttered noodles are good with this dish.