Heat a large nonstick skillet coated with cooking spray over medium-high heat. Sprinkle fish with 1/4 teaspoon salt and pepper. Add fish to pan; cook 5 minutes on each side or until fish flakes easily when tested with a fork. Remove fish from pan, and keep warm.
Add oil and shallots to pan; sauté 1 minute. Add mushrooms in a single layer; cook 2 minutes (do not stir). Cook an additional 2 minutes, stirring frequently. Add spinach; cook 30 seconds or until spinach wilts. Remove from heat; stir in 1/4 teaspoon salt, rind, and juice. Serve over fish.