Japanese tofu soup

Name: Japanese tofu soup
Prep time: 5 min
Cook time: 15 min
Total time: 20 min
Cuisine: Asian, Japanese
Ingredients: 200g fresh shiitake mushrooms, halved; 200g thick Korean glass noodles (see note) or thin glass noodles (bean thread vermicelli); 1/2 small Chinese cabbage, thickly sliced; 4 spring onions, chopped, plus extra to serve; 600g silken firm tofu (at room temperature), drained, cubed; 2L (8 cups) dashi (Japanese fish stock)(see note), made with dashi powder prepared to packet directions; 185ml mirin; 185ml soy sauce; 60ml sake