In a large skillet, heat the oil over medium heat, swirling to coat the bottom. Cook the spinach and lemon zest for 2 minutes, or until the spinach is wilted, stirring constantly. Transfer to a medium bowl. Stir in the bell peppers, basil, and walnuts. Let cool for 5 minutes.

Preheat the oven to 400°F. Line a baking sheet with aluminum foil. Lightly spray the foil with cooking spray.

Cut a lengthwise slit in the side of each fillet to make a pocket for the stuffing. Be careful to not cut through to the other side. With a spoon or your fingers, carefully stuff a scant 1/2 cup spinach mixture into each fillet. Transfer to the baking sheet. With a pastry brush or spoon, spread the mustard over the fish.

In a small bowl, stir together the remaining ingredients. Sprinkle over the fish. Lightly spray the tops with cooking spray.

Bake for 12 to 13 minutes, or until the fish flakes easily when tested with a fork and the filling is warmed through.

Nutrients per Serving: Calories 208; Total Fat 8.5 g; Saturated Fat 1.0 g; Trans Fat 0.0 g; Polyunsaturated Fat 3.5 g; Monounsaturated Fat 2.5 g; Cholesterol 65 mg; Sodium 280 mg; Carbohydrates 6 g; Fiber 1 g; Sugars 1 g; Protein 27 g
Dietary Exchanges: 3 lean meat; 1/2 starch