Fresh herbs and lemon flavor both the roasted potatoes and tender fish in this hearty one-dish meal. We tested with thick fresh halibut fillets, but you can substitute any other firm white fish fillets. Cut back on the baking time if your fillets are thinner than 1 inch.
Combine first 7 ingredients, stirring well. Place potato and onion in a 13 x 9-inch baking pan coated with cooking spray; toss with 2 tablespoons olive oil mixture. Bake at 400° for 35 to 40 minutes or until tender, stirring after 25 minutes.
While potato bakes, stir lemon rind and lemon juice into remaining olive oil mixture. Brush olive oil mixture evenly over fish; cover and let stand while potato finishes baking.
Arrange fish over potato in pan. Bake at 400° for 14 to 16 minutes or until fish flakes easily when tested with a fork.