Step 1

Preheat an oven to 375°F. Lightly butter an 8-by-12-inch gratin dish.

Step 2

In a saucepan over medium-low heat, melt the butter. Stir in the flour to make a roux and cook, stirring constantly, for 2 to 3 minutes; do not let the mixture color. Gradually whisk in the milk. Increase the heat to medium and bring the mixture to a simmer, whisking frequently. Adjust the heat to maintain a low simmer and cook, whisking frequently, until thickened into a white sauce, about 10 minutes. Whisk in the 1 tsp. salt and white pepper to taste. Cover and set aside.

Step 3

Place the spinach in a small stockpot and set over medium heat. Cover and cook until wilted, about 3 minutes. Empty into a colander and run cold water over the spinach to cool. In small handfuls, squeeze as much water from the spinach as possible. Using a chefs knife, chop the spinach finely. Transfer to a bowl and stir in 1/2 cup of the white sauce. Season with salt and pepper.

Step 4

Lay the sole fillets, smooth skin side down, on a cutting board. Check the fish for bones, removing and discarding any you find. Spoon one-fourth of the spinach mixture onto the center of each fillet, and fold the 2 ends of the fillet over the top of the spinach. Using a metal spatula, transfer each fillet, seam side down, to the prepared dish.

Step 5

In a blender, combine all but 1/4 cup of the shrimp, the remaining white sauce, the stock and the tomato paste and puree until smooth. Transfer to a bowl and season with salt and cayenne.

Step 6

Pour the sauce over the sole and bake until the fish is opaque throughout, about 25 minutes. Sprinkle the remaining shrimp on top of the fish and bake for 5 minutes more. Sprinkle with chives and serve immediately. Serves 4.

Step 7

Adapted from Williams-Sonoma Collection Series, Seafood, by Carolyn Miller (Simon & Schuster, 2005).