Preheat a dry skillet over medium heat. Add the halved onion and sliced ginger, and char them until blackened in spots, about 5 minutes per side. Remove and set aside.
In the same skillet, toast the star anise, cloves, cinnamon sticks, coriander seeds, and fennel seeds until fragrant, about 2-3 minutes. Transfer to a spice bag or cheesecloth and tie securely.
Place the beef bones and brisket in a large slow cooker. Add the charred onion, ginger, spice bag, salt, sugar, and water.
Cover and cook on low for 8 hours.
After 8 hours, remove the bones, brisket, onion, ginger, and spice bag from the slow cooker. Discard the solids.
Strain the broth through a fine mesh sieve into a large pot to ensure clarity.
Slice the brisket thinly and set aside.
Bring the broth to a simmer on the stovetop and adjust seasoning with additional salt or sugar if needed.
Prepare the rice noodles according to package instructions, usually by soaking in hot water until tender. Drain and divide among serving bowls.
Top noodles with slices of brisket and raw sirloin.
Ladle the hot broth over the noodles and beef slices to cook the raw sirloin slightly.
Serve pho with bean sprouts, Thai basil, cilantro, mint, lime wedges, jalapeño slices, hoisin sauce, and sriracha on the side for garnishing.