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, ½ 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.