Baked Haddock with Spinach and Tomatoes

Step 1
Preheat the oven to 375 degrees F (190 degrees C). Lightly butter a 2-quart casserole or baking dish large enough to hold haddock fillets in a single layer.

Step 2
Melt butter in a skillet over medium heat. Cook and stir onion in hot butter until soft, about 5 minutes. Stir in spinach and nutmeg; cook, stirring occasionally, for about 3 minutes.

Step 3
Arrange haddock fillets in the prepared baking dish. Spoon equal portions of spinach mixture between fillets. Spoon drained tomatoes around fillets and sprinkle with thyme, salt, and tarragon.

Step 4
Bake, uncovered, in the preheated oven until fish flakes easily with a fork, 20 to 25 minutes.

Step 5
While fish is baking, make sauce: Add enough water to reserved tomato juices to make 1 cup; set aside. Melt butter in the skillet over medium heat. Stir in onion and cook until soft, about 5 minutes. Pour 3/4 cup tomato juice into the skillet; bring to a boil. Whisk cornstarch into remaining 1/4 cup tomato juice, then add to the skillet. Reduce heat and simmer, stirring, until sauce thickens slightly.

Step 6
Pour sauce over baked fillets and serve.