We found that this betty, which consists of layered fruit, sugar, and breadcrumbs or cookie crumbs, is even better when served warm with low-fat vanilla ice cream.
Preheat oven to 350°.
Combine first 4 ingredients in a bowl, and toss gently.
Combine the breadcrumbs, cookie crumbs, and margarine; toss with a fork. Sprinkle 1/3 cup breadcrumb mixture in bottom of a 1 1/2-quart casserole coated with cooking spray. Top with half of plum mixture; sprinkle with 1/3 cup breadcrumb mixture. Top with remaining plum mixture; sprinkle with remaining breadcrumb mixture. Cover with lid; bake at 350° for 35 to 40 minutes. Uncover; bake an additional 15 minutes or until bubbly.