Place the dashi (stock), soy sauce, mirin, sweetener, and onion in a skillet and simmer gently over medium-low heat for about 5 minutes. Add the seitan pieces and continue to simmer gently, stirring occasionally until the onions are very tender. There should still be enough liquid in the pan to come up part way in the seitan. Puree the tofu, cornstarch and turmeric in a blender until smooth. Pour into the simmering mixture, but do not stir. Swish the pan around a bit to allow some of the liquid to come over the tofu.

Cover the pan and simmer until the tofu is firm to the touch, about 7 minutes. Divide the rice into 3 donburi (large, deep bowls). Lift out the mixture with a spatula and top each bowl with some. Pour additional sauce on top.