Combine the mayonnaise, 2 tablespoons lemon juice, and minced garlic in a small bowl.
Place breadcrumbs in a shallow dish. Combine 2 tablespoons lemon juice, milk, shallots, and mustard in a medium bowl. Dip 1 fish fillet in milk mixture, and dredge in breadcrumbs. Repeat the procedure with remaining fish. Place fillets on a broiler pan coated with cooking spray, and broil for 7 minutes on each side or until the fish flakes easily when tested with a fork.
Place fillets on bottom halves of buns; spread with 2 tablespoons mayonnaise mixture, and cover with bun tops.