Preheat oven to 250°F.
To assemble bouquet garni: Place one leek leaf on the counter. Top with bay leaf, celery stalk, parsley sprigs, thyme sprigs and tangerine (or orange) peel. Place the second leek leaf on top and tie the bundle together in four spots with kitchen string. Set aside.
To prepare stew: Heat 1 tablespoon oil in an ovenproof Dutch oven over medium-high heat. Add pancetta (or bacon) and cook until barely brown, 1 to 2 minutes. Transfer to a paper towel-lined plate, leaving any drippings in the pot.
Add beef in batches (do not crowd the pot) and cook until browned on all sides. Transfer to a large bowl and season with 1 teaspoon salt and 1/4 teaspoon pepper.
Add 1 tablespoon oil to the pot and add onions and garlic. Cook, stirring, until the onions are translucent, 3 to 4 minutes. Add carrots and cook, stirring, until they begin to soften, 4 to 5 minutes. Stir in tomato paste. Season with the remaining 1 teaspoon salt and 1/4 teaspoon pepper. Transfer the mixture to the bowl with the beef.
Reduce heat to medium and add the remaining 1 tablespoon oil to the pot. Add mushrooms and cook, stirring occasionally, until they are tender, 5 to 7 minutes. Transfer to a small bowl; set aside.
Pour wine into the pot and bring to a boil, scraping up any browned bits. Return the browned beef, the carrot mixture and the reserved pancetta (or bacon) to the pot along with the bouquet garni. Press down on the beef and vegetables, making sure to submerge them completely in the wine; if necessary, add just enough hot water to make sure they are covered. Cut a piece of parchment paper to fit the pot and press it directly on top of the stew, covering it completely.
Transfer the stew to the oven and cook, with the lid off, until the beef is tender enough to cut with a fork, about 3 hours. Check every hour to be sure the ingredients stay submerged in liquid during the entire cooking time. If too much wine evaporates, add a little hot water to make up for the loss. During the last 15 minutes of cooking, stir in the reserved mushrooms.
Remove and discard the bouquet garni. Combine chopped parsley and tangerine (or orange) zest in a small bowl and scatter on top of the stew just before serving.