Step 1

Adjust oven rack to upper position and preheat broiler on high.

Step 2

Bring a large stockpot filled with water to boil over high heat. Add the oxtail and boil for 10 minutes. Drain bones into a colander and rinse thoroughly with cold water. Placed bones in slow-cooker.

Step 3

Meanwhile, place onion and ginger on a foil-lined baking sheet and transfer to the oven. Cook until the ginger and onion achieve a nice char on both sides, about 8 minutes total. Remove from oven and add ginger and onion to slow-cooker.

Step 4

Add cinnamon, coriander seeds, fennel seeds, star anise, cloves and cardamom to a medium sauté pan over medium-high heat. Toast until fragrant, transfer to cheesecloth and seal into a packet. Place spices in the slow-cooker, along with water, beef base, fish sauce, sugar and 1 teaspoon salt. Set slow-cooker on lowest setting, allowing broth to cook for 10 hours.

Step 5

When soup is nearly done, par-cook noodles, stopping 30 seconds short of package directions. Drain and place in the bottom of four large, individual soup bowls.

Step 6

Discard spice pack and bones from broth. If there happens to be any scum or fat on the surface, skim and discard it. Taste broth and adjust seasoning, if necessary.

Step 7

Place slices of raw beef and shards of onions on top of noodles and ladle piping-hot broth into bowls; this should cook meat through and soften onions.

Step 8

Serve with lime, herbs, jalapeños, bean sprouts, Sriracha and hoisin.