For the beef: Heat a large pot over medium-high heat. Add unpeeled onion halves, cut side down, and cook without turning until blackened, 8-10 minutes. Transfer to a plate. Add 5 quarts water to pot, cover, and bring to a boil. Put one-third of the turnips, carrots, celery, parsley roots, and leeks into pot. Add meat, 4 of the marrowbones, whole peppercorns, and onions to pot and return to a boil. Partially cover pot and simmer, skimming foam that surfaces, until meat is very tender, 2 1⁄2-3 1⁄2 hours for the rump or 3 hours for the brisket.
Transfer meat and marrowbones to a dish and cover with plastic wrap to keep warm. Strain broth through a double layer of cheesecloth into a bowl, discarding vegetables. Return broth to pot and season to taste with salt. Add remaining turnips, carrots, celery, parsley roots, and leeks and simmer over medium-high heat until vegetables are just tender, 12-15 minutes. Remove vegetables from broth and cut into 1⁄2"-thick slices. Transfer to a dish and cover with plastic wrap to keep warm. Remove marrow from the reserved bones, discarding bones, and whisk into broth. Strain broth through a double layer of cheesecloth into a pot. Add remaining 6 marrowbones, cover, and keep hot over low heat.
For the creamed spinach: Bring a large pot of water to a boil over high heat. Stir in baking soda and generously season with salt. Add spinach and cook until wilted, 30-40 seconds. Drain spinach in a colander and rinse under cold running water until cool. Squeeze out excess water and set aside. Heat 6 tbsp. of the butter in a large skillet over medium heat until golden brown. Add spinach and stir until coated in butter and just warmed through. Add half the garlic, season to taste with salt and pepper, and cook for 1 minute. Pulse spinach in a food processor until finely chopped, then set aside. Melt remaining butter in a large skillet over medium heat. Add flour and cook, whisking constantly, for 1-2 minutes. Gradually add stock, whisking constantly, and continue to whisk until sauce is very thick, about 5 minutes. Add spinach and remaining garlic and cook, stirring frequently, until spinach is heated through, 1-2 minutes. Adjust seasonings. Transfer to a serving pot, cover, and keep warm over lowest heat.
For the potatoes: Peel and grate potatoes on the large holes of a box grater. Heat 2 tbsp. of the oil in a large skillet over medium-high heat. Cover bottom of skillet with half the potatoes and cook, undisturbed, for 1 minute. Scatter half the sliced onions over the potatoes, season to taste with salt, and cook, undisturbed, for 2-3 minutes. Stir mixture, then cook until potatoes are golden brown, about 7 minutes more, stirring mixture every 2 minutes or so. Transfer to a serving pot and keep warm over lowest heat. Repeat process with remaining oil, potatoes, and onions. Garnish with parsley just before serving.
For the apple-horseradish sauce: Steam apples in a covered steamer basket set over a pot of gently boiling water until soft, about 5 minutes. Using a fork, mash apples, horseradish, 1 tbsp. oil, 2 tsp. sugar, and salt to taste together in a bowl, then transfer to a serving dish and set aside to let cool.
For the white sauce: Soak bread in milk until soft, then squeeze out excess milk from bread, reserving milk. Using the back of a spoon, push bread and hard-cooked egg yolk through a sieve into a medium bowl. Add raw egg yolk, mustard, vinegar, pepper, a pinch of sugar, and salt to taste to bowl and whisk until smooth. Add 1⁄2 cup oil in a slow, steady stream, whisking constantly, then whisk in milk. Adjust seasonings. Transfer to a serving dish. Garnish with chives just before serving.
To serve, carve meat across the grain into 1⁄2"-thick slices and put into a large serving pot. Add marrowbones and vegetables and cover with broth. Season to taste with salt and garnish with chives. Serve remaining broth in soup bowls ladled over Shredded Crepes in Beef Broth as a first course, if you like. Serve meat, marrowbones, vegetables, and broth at the table along with the potatoes, creamed spinach, white sauce, and apple-horseradish sauce.