Preheat oven to 375°.
Combine pears and lemon juice in a 2-quart baking dish; toss gently to coat. Combine granulated sugar, cornstarch, and 1 teaspoon cinnamon; stir with a whisk. Add cornstarch mixture to pear mixture; toss well to coat.
Lightly spoon flour into a dry measuring cup; level with a knife. Place flour, 1/2 teaspoon cinnamon, brown sugar, and salt in a food processor; pulse 2 times or until combined. Add chilled butter; pulse 6 times or until mixture resembles coarse meal. Add oats and walnuts; pulse 2 times. Sprinkle flour mixture evenly over pear mixture.
Bake at 375° for 40 minutes or until pears are tender and topping is golden brown. Cool 20 minutes on a wire rack; serve warm or at room temperature.