- Simmer tendon in water for 30 minutes. Remove from water, cool, and slice into ¼-inch slices. Set aside.
- Preheat oven to 375 degrees. Season oxtail, beef neck, brisket, and marrow bone with salt and pepper and lightly coat with vegetable oil. Roast on a sheet tray until deeply caramelized, about 30 minutes.
- Place sliced tendon, roasted meats and marrow bone, thyme, and bay leaves in a large pot. Add enough cold water to cover by one inch. Bring to a boil then reduce to a low simmer. Simmer for 8 to 10 hours with the lid cracked. If mixture reduces by more than a third, add water.
- Remove and discard marrow bones, leaving marrow in broth. Remove oxtail and beef neck, pick meat from bones, and add meat back to broth. Discard bones. Remove brisket chunks, shred, and add back to broth.
- Bring broth to a boil then reduce to a low simmer, seasoning to taste with tamari, hoisin, salt, and pepper. Simmer for 20 minutes to let flavors meld. The broth should be rich and meaty.
- To serve, divide spaghetti evenly among soup bowls. Add a spoonful of sambal and pour piping-hot broth and meat over noodles. Top with scallions, serranos, and half an egg.