Make vinaigrette by combining the first 6 ingredients in a small bowl. Cover and set aside until needed.
Cut a piece of parchment paper into a round the size of the bottom of a large bamboo or stainless steamer. Poke a few holes in the paper, and then set in the steamer. Set the halibut in the steamer. Cover the steamer, set over simmering water, and steam until the fish is just cooked through, about 5 minutes.
While this occurs, bring 1/8 inch of water to boil in large skillet. Add the spinach and cooked until it barely wilts. Arrange the spinach on two heated plates. Top with the fish, spoon over the vinaigrette, and serve.