Rinse the marrow bones and beef shanks thoroughly under cold running water to remove any residual impurities. Transfer them to a large bowl, cover with fresh cold water, and allow them to soak for one hour. Drain, rinse again, and pat dry using paper towels. Season the beef shanks with salt and pepper, ensuring an even coating.

Preheat the oven to 425°F (218°C). Place the shucked corn on a rimmed baking sheet, drizzle with one tablespoon of oil, and roast for approximately 30 minutes, turning halfway through to achieve an even golden-brown color. Once roasted, allow the corn to cool before slicing off the kernels. Set the kernels aside and retain the cobs for broth enhancement.

In a large Dutch oven or stockpot, heat one tablespoon of oil over high heat until shimmering. Using tongs, carefully add the beef shanks and sear them for about five minutes, turning occasionally to develop a deep golden-brown crust. Transfer the seared shanks to a plate and set aside.

In the same pot, add garlic, onions, reserved corn cobs, and vegetable trimmings. Cover with water and bring to a rolling boil over high heat. Reduce to a gentle simmer and cook for 30 minutes to extract maximum flavor. Strain out the solids using a spider skimmer and discard.

Return the seared beef shanks, marrow bones, tendons, and knuckles (if using) to the pot. Ensure the beef is fully submerged in the broth, adding additional water if necessary. Bring to a boil, then reduce heat to low and let it simmer for 1.5 to 2 hours, partially covered, until the beef becomes fork-tender.

Once the beef has reached desired tenderness, stir in the Napa cabbage and roasted corn kernels. Cover the pot and allow it to cook for 10 minutes, until the cabbage is soft but not overcooked. Gradually season the broth with fish sauce, one tablespoon at a time, adjusting to taste.

While the soup is finishing, prepare the roasted elements. Toss the potatoes with the remaining two tablespoons of oil, salt, and pepper, then spread them evenly onto a lined baking sheet. Roast at 425°F (218°C) for 20 minutes, or until the exterior is crispy and golden brown.

For the marrow bones, arrange them on a separate baking sheet and roast at the same temperature for 20 minutes, or until the marrow is softened and slightly pulling away from the bone.

To serve, divide the marrow bones, beef shank, tendons, and knuckles among soup bowls. Top with the Napa cabbage, potatoes, and roasted corn kernels. Ladle the rich broth generously over each serving. Garnish with sliced scallions, and serve hot with steamed white rice on the side.