Choose the ripest Bartlett pears you can find, because their soft, juicy texture and sweet flavor make a big difference in this dessert.
Preheat oven to 375°.
Combine first 6 ingredients in a large bowl, stirring to combine. Add pears; toss gently to coat. Spoon pear mixture into a shallow 2-quart baking dish lightly coated with cooking spray.
Lightly spoon flour into a dry measuring cup; level with a knife. Combine flour, oats, brown sugar, cinnamon, and 1/8 teaspoon salt in a medium bowl. Cut in butter with a pastry blender or 2 knives until mixture resembles coarse meal, and stir in nuts. Sprinkle flour mixture over pear mixture.
Bake at 375° for 35 minutes or until topping is lightly browned. Cool on a wire rack 10 minutes.