Place rinsed beef marrow bones and brisket in a large stockpot, cover with cold water, and bring to a rolling boil for 5 minutes. Drain, rinse bones thoroughly under cool water to remove scum, and set aside.
Char onion and ginger directly over a gas flame or in a dry skillet until blackened and fragrant, about 3-4 minutes per side. Rinse lightly to remove loose black bits, keeping charred surface intact.
Toast star anise, cinnamon, cloves, coriander seeds, and peppercorns in a dry skillet over medium heat until fragrant, about 2-3 minutes. Immediately transfer to a spice bag or cheesecloth and tie closed.
Add cleaned bones, brisket, charred onion, ginger, spice bag, fish sauce, rock sugar, salt, and 10 cups cold water to the Instant Pot. Secure lid, set valve to sealing, and pressure cook on HIGH for 45 minutes.
Allow natural pressure release for 15 minutes, then quick release remaining pressure. Carefully open lid, remove brisket, and discard spice bag, onion, and ginger. Strain broth through a fine-mesh strainer.
Taste and adjust broth with additional fish sauce, sugar, or salt as needed. Skim excess fat if desired, then keep broth at a gentle simmer while preparing bowls.
Cook rice noodles according to package directions, drain, and divide among serving bowls. Slice raw beef paper-thin against the grain and slice cooked brisket. Arrange fresh herbs and toppings on a platter.
Warm serving bowls, then divide noodles between them. Top with cooked brisket slices and arrange raw beef slices on top. Ladle boiling broth over the meat and noodles, ensuring it covers everything.
Garnish with green onions and white onion slices. Serve immediately with bean sprouts, Thai basil, cilantro, lime wedges, jalapeño, hoisin sauce, and sriracha on the side for each person to customize.