How to Make It
In a medium saucepan, melt the butter. Add the ham, onion, garlic and ginger and cook over moderately high heat until the onion is starting to brown, about 5 minutes. Add the stock, fish sauce, miso, soy sauce and ½ cup of water and bring to a boil. Simmer, uncovered, until the broth is flavorful, about 20 minutes. Strain the broth and discard the solids. Return the broth to the pan and keep warm.
Bring a large, deep skillet of water to a simmer over moderate heat; add the vinegar. One at a time, break the eggs into a small bowl and slide them into the simmering water, leaving plenty of space between them. Poach the eggs until the whites are set and the yolks are still runny, about 4 minutes. Using a slotted spoon, carefully transfer the eggs to a paper towel–lined plate.
Bring a medium saucepan of water to a boil over high heat. Add the noodles and cook until al dente. Drain and transfer the noodles to large soup bowls. Top with the poached eggs. Ladle the hot broth on top and serve with the ham, cheese, avocado, scallions and nori.