In a saucepot, bring the dashi or vegetable broth (plus water if using) to a gentle boil over medium heat.
Add the cubed tofu, sliced mushrooms, and chopped leafy vegetables to the boiling broth; simmer briefly until the mushrooms and greens are tender, about 1–2 minutes.
Slowly pour the whisked egg into the simmering soup while stirring gently to create thin ribbons; cook for about 1–2 minutes.
Remove the pot from the heat.
If using Miso & Easy: stir the Miso & Easy into the pot until incorporated, then top with chopped green onions and serve immediately.
If using miso paste: ladle about 1/2 cup of the hot broth into a bowl, whisk in the miso paste until smooth, then pour the mixture back into the pot and stir gently. Top with chopped green onions and serve immediately.