In a pot, bring the dashi to a simmer and add the soy sauce and mirin.
Add the chicken, kamaboko, and shiitake mushrooms to the pot and simmer until the chicken is cooked through.
Meanwhile, broil the mochi until it puffs up and turns golden brown.
Place a piece of mochi in each serving bowl.
Add the spinach to the pot and cook until wilted.
Ladle the soup over the mochi in each bowl.
Garnish with a slice of yuzu or lemon and serve hot.