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 degrees. 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 12-13 minutes, or until the fish flakes easily when tested with a fork and the filling is warmed through.
All materials courtesy of ECES; recipes are reprinted with permission from Light & Easy Recipes, 2008, by the American Heart Assoc. For more information about Go Red For Women, visit www.GoRedForWomen.org