In a medium saucepan, add 560 ml dashi (Japanese soup stock), 1 Tbsp mirin, 1 tsp sugar, 1.5 Tbsp soy sauce, and 0.13 tsp Diamond Crystal kosher salt.
Bring the broth to a gentle simmer. Once simmering, remove the pot from the heat. Cover with a lid and set aside.
Bring a large pot of water to a boil. When the water comes to a full rolling boil, add 2 servings udon noodles and cook according to the package instructions. The brand of frozen udon noodles I use requires reheating them for 1 minute. Using chopsticks, stir the noodles to loosen them up.
When the noodles are done, drain them in a colander or scoop them up with a fine-mesh strainer. Serve the hot udon noodles in individual bowls.
Pour the hot broth over the noodles and top with thinly sliced green onion and shichimi togarashi for a spicy kick (optional). For a more substantial lunch or dinner, you can top it with tempura, beef, fish cake and toasted mochi, tofu, Inari age, or vegetables.
