1. To prepare the fish, combine first 7 ingredients (through wasabi paste). Place 3/4 cup shallot mixture in a large zip-top plastic bag. Set remaining shallot mixture aside. Add fish to bag; seal and marinate in refrigerator 30 minutes, turning once.
2. To prepare ginger-garlic mayonnaise, combine mayonnaise and next 5 ingredients.
3. Preheat broiler.
4. Remove fish from bag; discard marinade. Place fish on a broiler pan; broil 10 minutes or until fish flakes easily when tested with fork.
5. Combine reserved shallot mixture and sesame oil in a large bowl. Add spinach; toss well.
6. Spread about 1 tablespoon ginger-garlic mayonnaise evenly over each of 4 bread slices; top each with 1 fillet and about 1 cup spinach mixture. Top with remaining bread slices
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