Preheat the oven to 400F.
Arrange the beef shank bones on a tray lined with foil.
Roast the bones until lightly caramelized.
Transfer the roasted bones to the slow cooker and add enough water to cover.
Throw in the cloves, coriander seeds, cinnamon bark, star anise, garlic, onion, ginger and lemongrass.
Drizzle in two tablespoons fish sauce.
Cook the bones on LOW for 16 to 18 hours.
Cool the bones and broth.
Carefully scoop out the bones and transfer to a container that can go into the fridge.
Strain the broth directly into the container with the bones.
Chill overnight to allow the flavors to deepen.
Reheat the bones and broth. Taste and add more fish sauce, if needed.
Place the cooked noodles in bowls.
Divide the tendon among the three bowls and add a bone at the center (remember, that contains marrow).
Garnish the beef tendon noodle soup with cilantro, scallions and fried shallots.