In a large non-tick skillet, heat the oil over medium-high heat. Add the sliced onions and cook until just beginning to color, about 6 minutes. Add the lemongrass, cilantro root and garlic and cook for 30 seconds. Add the lobster bodies, star anise, water, carrots, ginger, cinnamon and peppercorns 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. Remove stock from the stove and strain through a fine strainer. Place in a new stockpot and bring to a simmer.
Meanwhile, bring a pot of water to a boil. Add the noodles to the water and boil until just tender, then drain in a colander.
To assemble the Pho, bring the soup to a simmer, as needed, and add the fish sauce. Drain the boiled noodles in a colander and divide among 4 soup bowls. Divide the remaining ingredients among the bowls and ladle the broth over each portion at the table. Garnish with lime wedges, mint and bean sprouts and serve immediately.