Place fillets in an 11- x 7- x 1 1/2-inch baking dish coated with cooking spray; brush fish with lemon juice. Bake at 400° for 12 minutes.
While fish bakes, combine chives, mayonnaise, and mustard, stirring well.
Spread mayonnaise mixture evenly over fish. Bake 3 additional minutes or until fish flakes easily when tested with a fork. Garnish with lemon slices, if desired.