• Before You Start: Please note that this recipe calls for homemade dashi that takes a minimum of 20 minutes to make. Please see my recipe for How to Make Dashi. If you don‘t have time to make this Japanese soup stock from scratch, you can use a dashi packet or dashi powder.
• I recommend prepping this recipe the day before you plan to serve. The next day, reheat the soup and add the mochi just before serving. For more helpful tips on planning your Japanese New Year feast, please read my A 5-Day Osechi Cooking Timeline blog post.
• Gather all the ingredients.
  

To Prepare the Ingredients

• Cut 1 chicken thigh into bite-sized pieces (1 inch, 2.5 cm) and put them in a small bowl. Then, add ¼ tsp Diamond Crystal kosher salt.
  
• Add 1 Tbsp sake and mix well with your hands. Let the chicken marinate for 15 minutes.
  
• Meanwhile, blanch 4 stalks komatsuna (or spinach) in boiling water and cook until tender (do not overcook). Remove from the pot and soak it in iced water to stop the cooking.
  
• Squeeze the water out of the komatsuna and cut it into pieces 1½ inches (3 cm) long. Set aside.
  
• Make a knot in the stem of each of 4 sprigs mitsuba (Japanese parsley) (optional). Set aside for garnish. Next, peel 2–3 thick strips of yuzu zest (optional). 
  
• Flip the pieces over and remove any white pith. Then, slice the strips of yuzu zest into very thin julienned strips. Place on a small dish and set aside to serve as garnish later.
  

To Cook the Soup

• Add 4 cups dashi (Japanese soup stock) to a large saucepan or pot and bring it to a boil. Once boiling, add the chicken pieces.
  
• Cover to cook for 5–7 minutes, depending on the size of the chicken pieces.
  
• Once the chicken is cooked, skim off the foam and fat from the surface of the soup with a fine-mesh strainer.
  
• Add 1 Tbsp sake, 1 Tbsp soy sauce, and 1 tsp Diamond Crystal kosher salt. Stir well and keep it covered on a simmer.
  
• Once the soup broth and toppings are ready, place 4 pieces Japanese rice cake (kiri mochi) directly on the rack of a toaster oven and toast until puffy. You can also toast the mochi over an open flame, under a broiler, or in a frying pan.
  

To Serve

• Serve the chicken and soup in individual serving bowls. Add one piece of toasted mochi on top of the soup in each bowl. Top with the komatsuna, mitsuba (optional) and yuzu (optional). Enjoy immediately.
  

To Store

• Remove the mochi and store the leftover soup in an airtight container or in the pot; keep in the refrigerator for up to 3 days. I recommend toasting and adding the mochi to the soup just prior to serving.