Cut fish into 4 equal pieces; place in a shallow glass bowl. Mix sake, soy sauce, ginger juice, and salt in a small bowl, then pour over the fish. Let stand 10 minutes; drain, reserving marinade.
Cut carrot into 1/8 inch thick slices; if desired, cut slices into decorative shapes using knife or Japanese vegetable cutter. Cut peppers crosswise into 1/8 inch thick slices. Cut onion crosswise into 1/8 inch thick slices and separate slices into rings.
Brush oil over center of 4 pieces (10 inches square) aluminum foil. Place 1 piece fish on the center of each square. Arrange sliced vegetables over the fish. Sprinkle with reserved marinade.
Bring two opposite sides of foil together over center of food; fold together in 1/2 inch folds to seal. Fold up each end of the foil packet to securely seal in fish and vegetables.
Place packets on a baking sheet and bake 15-20 minutes, or until fish is done. Serve immediately with lemon slices.