Heat basting oil in braising pan on MEDIUM-HIGH. Add sausage, breaking up with a spoon; cook 4-5 min, until slightly browned (about 75% cooked). Add celery and onion; cook, stirring, 5 min. Add butter and stock; bring to a simmer. Turn off heat.
Combine croutons and apple; gradually add to mixture in braising pan, stirring gently. Season to taste with salt and pepper.
Bake on center rack of oven 30-35 min or until stuffing is nicely browned and cooked through.