COOK and stir onions in butter in medium skillet on medium heat 5 min. or until crisp-tender. Add cream cheese and milk; cook on low heat 5 to 8 min. or until cream cheese is completely melted and sauce is well blended, stirring occasionally. Remove from heat. Stir in horseradish.
FILL pita pockets with meat, tomatoes and lettuce.
DRIZZLE with horseradish sauce.