For the dressing: Combine all of the ingredients in a small bowl. Cover and refrigerate until ready to use.
For the salad: Blanch the seaweed in a large saucepan of boiling salted water for 2 minutes. Drain. Transfer to a bowl. Bring soy sauce, vinegar, mirin, and sugar to a boil in a small saucepan, stirring to dissolve the sugar. Pour the mixture over the seaweed. Mix in the sesame seeds and jalapeno. Cool completely. Drain.
For the tuna: Mix the first 5 ingredients in a large bowl. Add the Ahi and toss to coat. Let stand 1 hour at room temperature. Preheat the broiler. Place the Ahi on a foil lined baking sheet. Broil until rare, about 1 minute per side. Arrange the seaweed salad on plates. Top with Ahi. Ladle over the dressing and serve.
Recipe courtesy of David Rosengarten