For the broth:
2 beef shanks with meat on them (about 2 1/4 pounds total)
3 large yellow onions, halved
1 6-inch piece ginger, halved lengthwise
1 head garlic, halved crosswise
2 tablespoons vegetable oil
Kosher salt
2 cinnamon sticks
4 star anise pods
3 tablespoons black peppercorns
3 quarts low-sodium beef broth
1/3 cup fish sauce
For the soup:
1 pound flat rice noodles
8 ounces deli-sliced roast beef, torn into pieces
4 cups bean sprouts
1 large bunch Thai basil or mint, leaves torn
2 jalapeno peppers (red and/or green), thinly sliced
1 large red onion, thinly sliced
Hoisin sauce and/or Sriracha (Asian chile sauce), for serving