PREHEAT oven to 325°F. Cook onion and celery in margarine in large skillet on medium-high heat until crisp-tender. Add 2 Tbsp. of the mustard, the bread cubes, apple and poultry seasoning; mix well. Set aside.
UNROLL roast. Spread with 2 Tbsp. of the remaining mustard; top with the bread mixture. Re-roll roast; secure with kitchen string. Place on rack in roasting pan.
BAKE 2 hours or until meat thermometer inserted into thickest part of roast registers 160°F. Remove roast from pan to carving board. Let stand 10 minutes before slicing to serve. Meanwhile, remove 1 Tbsp. of the drippings from roasting pan; place in small saucepan. Add flour; stir with wire whisk until well blended. Cook on medium heat 1 minute, stirring constantly. Add apple juice and remaining 1/4 cup mustard; cook and stir until mixture thickens and begins to boil. Remove from heat; stir in parsley. Slice the roast. Serve topped with mustard sauce.