PREHEAT oven to 350°F. Mix sour cream, mayo, dill, lemon juice and mustard until well blended. Cover and refrigerate until ready to serve.

HEAT large skillet on medium-high heat. Meanwhile, rinse spinach under cold running water. Add to hot skillet (do not drain first); cook until wilted, stirring frequently. Place spinach in colander; press with back of large spoon to remove excess liquid. Transfer spinach to large bowl. Add cracker crumbs and cream cheese spread; mix well. Set aside. Cut each fish fillet crosswise in half. Place one half of each fillet in single layer in greased 13x9-inch baking dish; top evenly with the spinach mixture. Cover with the remaining fish fillets. Sprinkle with salt and pepper.

BAKE 15 to 20 min. or until fish flakes easily with fork. Place on serving plate; top with the sour cream mixture.