Add water and the dashi packet to a small pot. Bring to a boil over medium heat, then simmer for 3–5 minutes. Remove and discard the dashi packet.
Add 2 tablespoons usukuchi soy sauce to the broth. Taste and adjust with more soy sauce or a small pinch of salt as needed.
Microwave the frozen udon according to the package instructions — usually about 4 minutes per portion at 500–600W.
Warm the simmered age in its broth or briefly in the microwave (20–30 seconds).
Place hot udon in each bowl. Ladle the hot broth over the top. Top with a piece of warmed simmered age and a generous sprinkle of green onion. Serve immediately.