Preheat the grill.

In a large non-stick skillet, heat the oil over medium-high heat. Add the sliced onions and cook until just beginning to color, about 6 minutes. Add the garlic and cook, stirring, for 30 seconds. Remove from the heat.

Press three star anise into the flesh of the halved onions and place on the grill. Grill until blackened, about 8 minutes. Remove from the heat.

Add the bones to a large stockpot and cover with water by 2 inches and bring to a boil. Reduce the heat to medium-low and simmer for 15 minutes, skimming the top to remove any foam that forms on the surface. Add grilled and Sauteed onions, and the remaining ingredients, stir well, and simmer partly covered for at least 6 and up to 10 hours, adding more water as needed to keep the bones covered by 1-inch. Strain through cheesecloth or a fine mesh strainer. Remove meat from bones and set aside. Discard everything but the meat from the bones and the broth.

Meanwhile, bring a pot of water to a boil, and heat enough vegetable oil in a large saucepan to come 4 inches up the sides. Add half of the noodles to the water and boil until just tender, then drain in a colander. Add the remaining noodles to the hot oil and fry briefly until golden brown and crisp, then transfer to paper towels to drain.

To make the Pho, bring the soup to a simmer, as needed, and add the fish sauce. Transfer to a large tureen or pot that can be placed on the table and kept warm. Drain the boiled noodles in a colander and divide among 6 soup bowls. Divide the remaining ingredients and the reserved meat from the bones among the bowls and ladle the broth over each portion at the table. Garnish with lime wedges and serve immediately.