Place mushrooms in a medium bowl and pour over boiling water. Allow to soak for 20 minutes, then strain liquid into a saucepan. Transfer shiitakes to a cutting board. Slice mushrooms and remove stems.
Heat the saucepan of mushroom stock over medium heat to bring to a simmer. Adjust heat as needed to maintain a simmer.
Heat sesame oil in a Dutch oven or heavy pot over medium heat. Add onion, garlic, and ginger and cook briefly until fragrant. Add mushrooms, 1/2 tsp salt, and 1/4 tsp pepper to the pot and cook, stirring occasionally, until onions are starting to soften, about 2-3 minutes.
Add Arborio rice and cook for 1-2 minutes while stirring. Pour mirin into the pot and stir until it is mostly absorbed, scraping any bits stuck to the bottom of the pan.
Add mushroom stock to pot ½ cup at a time, still on medium heat, and stirring often. Add more liquid once most of the last addition has been absorbed. After about 15 minutes, or when 3 cups broth have been added, start checking to see if the rice is cooked (it should be al dente), if not continue to add more broth as needed. Heat additional water if you use all of the mushroom stock and rice is not finished cooking yet. With the final addition of liquid, also add in kale, Parmesan cheese, and rice wine vinegar. The risotto should not hold form, add more liquid if needed. Remove from heat.
Divide risotto amongst bowls and top with sliced scallion.