Heat oil in a large nonstick skillet over medium-high heat.
Add carrot; saut 2 minutes or until tender.
Add garlic; saut 15 seconds.
Add 1/4 teaspoon salt and spinach; saut 2 minutes or until spinach wilts.
Combine 1/4 teaspoon salt, green onions, and next 4 ingredients (through grated ginger).
Fold 4 (16 x 12-inch) sheets heavy-duty foil in half lengthwise. Open foil; place about 1/2 cup spinach in center of each foil sheet. Top each with 1 fillet.
Drizzle mirin mixture evenly over fillets; sprinkle evenly with black pepper. Fold foil over fillets; tightly seal edges.
Place foil packets in a single layer on a jelly-roll pan.
Bake at 400 for 20 minutes or until fish flakes easily when tested with a fork.