Preheat a skillet with a lid or a pot to medium heat with a drizzle of oil. Thinly slice the bok choy leaves, separating the whites and greens.

Add the whites of the bok choy to the pot with the dashi, soy sauce, and mirin. Stir, and cover. Let cook for 5 minutes until slightly cooked.

Add the greens and the tofu to the pot. Cover, and cook for 5 minutes more until the liquid has been partially absorbed and the greens are soft.

Taste, an add more soy sauce if you'd like. Serve with white rice.