Preheat oven to 400'. Heat oil over medium-high heat; add onion and garlic. Cook and stir 2-3 minutes or until onion is tender. Stir in pasta sauce, tortellini, chicken, milk, water, peas and black pepper. Heat until mixture just comes to a boil; remove from heat. Stir basil into pasta mixture.
Meanwhile, for crumb topping grate Parmesan cheese. Place butter in microwavable dish and melt. Stir in bread crumbs and cheese; mix well.
Spoon pasta mixture into a square casserole dish, then sprinkle with crumb topping. Bake 15-20 minutes or until edges are bubbly and topping is golden brown.