As a short-cut, use leftover tempura or pre-made tempura from a Japanese deli or supermarket. To crisp leftover or pre-made tempura, simply reheat in a dry non-stick pan (do not coat the pan with any oil) over medium-high heat, turning over frequently until crisp.
Set tempura aside.
In a medium pot, bring water to boil and cook frozen udon noodles for 1 to 2 minutes until tender. Drain the noodles well. Separate noodles into two portions and serve in large Japanese-style soup bowls.
Next, make the udon soup. In a separate medium pot, combine water and dried dashi powder mixture (or use homemade dashi using the recipe available here) over medium-high heat. Next, add soy sauce and mirin and bring to a boil. Taste the udon soup. Optionally, a teaspoon of sugar may be added but is not necessary. Adjust the taste of the broth if necessary, by adding more soy sauce if desired. Turn off heat.
Ladle hot broth over the udon noodles, top with crisp tempura. Garnish with green scallions and fish cake. Add Japanese 7-chili spice (shichimi togarashi).