Preheat oven to 425°.
Combine first 4 ingredients, stirring with a whisk. Set aside.
Fold 4 (16 x 12-inch) sheets of heavy-duty aluminum foil in half crosswise. Open foil; layer 2 tablespoons carrot, 1 tablespoon onions, 1/4 cup sugar snap peas, 1/4 cup mushrooms, and 1/4 cup bell pepper on half of each foil sheet. Dip fillets into soy mixture, turning to coat evenly. Place one fillet on each mound of vegetables. Spoon remaining soy mixture evenly over each serving. Fold foil over halibut; tightly seal edges. Place packets on a baking sheet. Bake at 425° for 13 minutes. Remove from oven; let stand 3 minutes.
While packets bake, cook soba according to package directions, omitting salt and fat. Drain and rinse under cold water; drain. Place 1 cup soba on each of 4 plates. Top each serving with contents of 1 packet. Sprinkle each serving with 1 tablespoon cilantro. Serve immediately.
Wine note: Soy, which is salty, and jalapeños, which have a fiery heat, both need a fruity wine to cushion their impact. A California riesling works well. Most California rieslings are very fruity and less crisp than their European counterparts. Try Jekel Riesling 2005 from Monterey ($13). --Karen MacNeil