Place all beef bones in a large stockpot, cover with cold water, and bring to a boil. Boil for 10–15 minutes until gray foam rises, then drain, rinse each bone thoroughly under cold water, and clean the pot.
Preheat oven to 450°F (230°C). Arrange blanched bones on a rimmed baking sheet and roast for 30–40 minutes, flipping halfway, until deeply browned. Simultaneously roast the carrots, halved onion, and garlic on a separate baking sheet for 20 minutes until caramelized.
Transfer roasted bones and vegetables to the clean stockpot. Add celery, bay leaves, peppercorns, herb stems, and apple cider vinegar; let sit 20–30 minutes, then cover with 10–12 cups cold water.
Bring the pot to a gentle boil, skimming foam from the surface, then reduce heat to the lowest setting and barely simmer (just a bubble or two) for 8–12 hours, partially covered, adding water if the level drops below the bones.
Cool slightly, then strain the broth through a fine mesh strainer into a large bowl, discarding all solids. Remove excess fat using a fat separator or by refrigerating overnight and lifting off the solidified fat. Season generously with salt.
Pat beef cubes dry, season with salt and pepper, then sear in olive oil over medium-high heat in batches (2–3 minutes per side) until deeply browned on all sides. Transfer to a plate.
In the same pot, cook diced onion for 4–5 minutes until softened, add garlic for 1 minute, then stir in tomato paste and cook 2 minutes until darkened. Add dried thyme and smoked paprika and stir 30 seconds until fragrant.
Return browned beef to the pot, pour in 6–8 cups of bone broth, scraping up browned bits, then add diced tomatoes. Bring to a boil, reduce to a gentle simmer, cover partially, and cook 30 minutes. Add carrots, celery, and potatoes and simmer uncovered 20–25 more minutes until vegetables and beef are fork-tender.
Taste and adjust seasoning with salt, pepper, or a small splash of apple cider vinegar to brighten the flavor. Ladle into bowls and garnish with freshly chopped parsley.