Sprinkle fish with thyme, pepper, and salt. Heat a large nonstick skillet over medium-high heat. Coat pan with cooking spray. Add fish; cook 1 minute on each side. Add broth, and bring to a boil. Cover, reduce heat, and simmer 5 minutes. Add mushrooms; cook, uncovered, 1 minute or until mushrooms are tender. Remove fish from pan; keep warm.
Add cream and mustard to pan; stir with a whisk until well combined. Cook 1 minute or until thoroughly heated. Serve sauce over fish.