• Gather all the ingredients. Start bringing a big pot of water to a boil over medium-high heat.
  

To Prepare the Udon Broth (with concentrated mentsuyu)

• In a medium saucepan, combine 2⅓ cups water, ⅓ cup mentsuyu (concentrated noodle soup base), and 1 Tbsp mirin and mix well. Bring the mixture to a gentle simmer over medium heat. Cover with a lid and turn off the heat. If you do not have mentsuyu, see the instructions below for how to cook udon broth from scratch.
  

To Prepare the Toppings

• Using a toaster oven, standard oven, frying pan, or stovetop Japanese fish grill (I purchased this from a Japanese market), toast both sides of 2 pieces Japanese rice cake (kiri mochi) until they are browned and puffed up. Set them aside.
  
• Meanwhile, prepare the toppings. Cut 4 slices kamaboko (fish cake).
  
• Cut 1 green onion/scallion into thin rounds. Set aside. I also defrosted some yuzu zest (optional; here‘s how I freeze yuzu peels).
  
• When the water is boiling, blanch 2 oz spinach with the stem side down into the water first, for 30–45 seconds.
  
• Remove the spinach from the pot (and shock it in iced water to stop the cooking and bring out the bright color—but this step is optional). Squeeze the water out and cut the spinach into 2-inch (5-cm) pieces. Set aside.
  

To Cook the Udon Noodles

• In the same pot of boiling water, cook 2 servings udon noodles according to the package instructions. For frozen udon noodles, cook for 1 minute. Drain the udon noodles completely.
  

To Serve

• Transfer the udon noodles to individual bowls and pour udon broth to completely cover the noodles.
  
• Top the noodles with the toasted mochi, blanched spinach, chopped green onions, and yuzu zest (optional). Sprinkle with optional shichimi togarashi (Japanese seven spice) for a spicy kick.
  

To Store

• You can keep the toppings and udon broth in separate airtight containers and store them in the refrigerator for 3 days. However, I highly recommend cooking the udon noodles and toasting the mochi right before you serve.

To Make the Udon Broth from Scratch (without Mentsuyu)

• In a medium saucepan, add 2½ cups water and 1 dashi packet. Bring it to a boil over medium heat. If you want to make dashi from scratch, follow my very easy tutorial How to Make Dashi. If you‘re vegan/vegetarian, make my Kombu Dashi or a combination of kombu dashi and Shiitake Dashi.
  
• Simmer for 2–3 minutes. Then squeeze the liquid from the dashi packet and discard the packet.
  
• Add 1½ Tbsp soy sauce, 1 Tbsp mirin, 1 tsp sugar, and ⅛ tsp Diamond Crystal kosher salt and mix it all together. Cover with a lid (don‘t let the soup evaporate) and remove it from the heat. Set aside. The udon broth is ready to use.