Add the water to a medium saucepan and bring to a boil o over medium-high heat. Add the dashi packets (see note) to the pot, cover, and boil for 2-3 minutes. Remove the packets.
Lower the heat to medium and whisk in the miso paste until it's fully incorporated.
Reduce the heat to a low simmer and add the tofu and green onions. Simmer for a few minutes to warm the tofu before portioning into bowls and serving warm.
Notes
Most dashi powder packets in the stores near me call for 600 mL of water (2.5 cups) per packet of dashi. If you're using dashi power (not in a packet or stick), whisk it into the boiling water until dissolved.