Combine first 6 ingredients, stirring with a whisk; pour into a 10-inch pie plate coated with cooking spray.
Open a small metal vegetable steamer; place steamer upside down in a large, deep wok. Add water; bring to a simmer. Wearing oven mitts, carefully place pie plate on top of inverted steamer. Cover and cook 3 minutes. Carefully remove wok lid; sprinkle custard with prosciutto and onions. Cover and cook 3 minutes or until a knife inserted in center comes out clean. Wearing oven mitts, carefully remove pie plate from wok. Drizzle custard with oil; cut into 4 wedges. Serve over rice.