Roast the Bones: Preheat your oven to 400°F (200°C). Place the beef soup bones on a baking sheet and roast for 30-40 minutes, until the bones are nicely browned.
Brown the Meat: While the bones are roasting, heat a large pot over medium-high heat. Add the beef chuck roast or short rib pieces and brown them on all sides, about 5 minutes total. Remove the meat from the pot and set aside.
Make the Broth: Transfer the roasted bones to the pot and add the onion, carrots, celery, and garlic. Pour in the beef broth and add the bay leaves and thyme. Bring the mixture to a boil, then reduce the heat and let it simmer for 2-3 hours, skimming off any foam or fat that rises to the surface.
Extract the Marrow: Using tongs, carefully remove the bones from the pot. Use a spoon to scoop out the delicious bone marrow from the center of the bones and set it aside.
Finish the Soup: Return the broth to the pot and bring it back to a simmer. Add the browned beef pieces and let them cook for an additional 30 minutes, until very tender. Season with salt and pepper to taste.
Serve: Ladle the hot soup into bowls, making sure to include some of the tender beef pieces. Top each serving with a generous spoonful of the roasted bone marrow. Garnish with chopped parsley before serving.