HEAT oven to 400°F. Cook onions in dressing in large covered skillet on medium-low heat 25 min. or until tender, stirring occasionally. Remove from heat; set aside to cool.
MEANWHILE, roll pastry into 12-inch square on lightly floured surface. Place in parchment-lined 15x10x3/4-inch pan. Spread with cream cheese to within 1 inch of edges; top with remaining ingredients.
BAKE 15 min. or until pastry is golden brown. Serve warm or cooled to room temperature.