HEAT oil in medium saucepan on medium-high heat. Add onions; cook and stir 3 to 4 min. or until crisp-tender.
STIR in Neufchatel, milk and seasonings; cook on low heat 1 to 2 min. or until Neufchatel is melted and sauce is well blended.
ADD 1/3 of the spinach; cook 2 min. or just until wilted, stirring frequently. Add remaining spinach in batches, cooking and stirriing 1 to 2 min. after each addition.