1. Peel sweet potatoes; cut into 1-inch cubes. In 2-quart casserole, place sweet potatoes and 2 tablespoons water; cover with casserole lid or large plate. Cook at high (100% power) 4 minutes, stirring after 2 minutes.
2. Meanwhile, peel and core apples and cut into 1-inch pieces. Add apples, brown sugar, butter or margarine, salt, cinnamon, and nutmeg to sweet potatoes in casserole; stir gently to combine. Cover and cook at high 10 to 12 minutes until potatoes and apples are fork-tender, stirring gently with rubber spatula after 6 minutes.
3. In cup, stir cornstarch and 1 teaspoon water until smooth; stir into liquid in casserole. Cover and cook at high 1 minute or until liquid thickens slightly.