Cook the noodles in a large saucepan of boiling water for 8 minutes or just until tender. Drain.
Divide the noodles among serving bowls. Top with the prawns and tofu.
Place the water and miso in a medium saucepan over medium-high heat. Bring to the boil, whisking often, until the miso dissolves.
Reduce heat to low. Add the ginger and soy sauce to the pan. Simmer for 2 minutes. Add the asparagus, mushrooms and spinach, and simmer for 30 seconds or until the spinach just wilts.
Ladle the soup over the noodles. Sprinkle with shallot and serve immediately.