PREHEAT oven to 400°F. Cook leeks, spinach and thyme in dressing in large skillet on medium heat 5 to 6 min. or until leeks are tender, stirring frequently.
LAYER sweet potatoes, leek mixture, 1/2 cup of the shredded cheese and baking potatoes in greased 2-L baking dish. Mix milk, salt and pepper; pour over vegetables. Sprinkle with remaining 1/2 cup shredded cheese and the Parmesan cheese. Cover.
BAKE 45 min.; uncover. Bake an additional 15 min. or until vegetables are tender and top is lightly browned.