Wasabi Salmon Sandwiches

To prepare fish, combine first 7 ingredients. Place ¾ 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. To prepare ginger-garlic mayonnaise, combine mayonnaise and next 5 ingredients (through garlic clove). Preheat broiler. Remove fish from bag; discard shallot mixture in bag. Place fish on broiler pan; broil 10 minutes or until fish flakes easily when tested with a fork. Combine the reserved shallot mixture and dark sesame oil in a large bowl. Add spinach and toss well. Spread about 1 tablespoon ginger-garlic mayonnaise evenly over each of 4 bread slices; top each bread slice with 1 salmon fillet. Top each fillet with about 1 cup spinach mixture; top with remaining bread slices.