Combine first 4 ingredients in a small saucepan; bring to a boil. Reduce heat, and simmer, uncovered, until reduced to 3/4 cup (about 20 minutes). Discard bay leaf. Combine milk and cornstarch, stirring with a whisk. Add to broth mixture; bring to a boil. Cook for 1 minute, stirring constantly. Stir in watercress; cook for 30 seconds. Keep warm.
Arrange the salmon fillets in a single layer in an 11 x 7-inch baking dish, and sprinkle with lemon juice, salt, and pepper. Cover with plastic wrap, and vent. Microwave at high 4 1/2 minutes or until fish flakes easily when tested with a fork. Place 1 fillet on each of 4 plates; top each serving with 1/4 cup sauce. Garnish with watercress sprigs and lemon wedges, if desired.