1. Preheat oven to 375°F.
2. Combine apples, 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.
3. 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 is coarse. Add oats and walnuts; pulse 2 times.
4. Sprinkle flour mixture evenly over pear mixture. Bake at 375°F for 40 minutes or until pears are tender and topping is golden brown. Cool 20 minutes; serve warm or at room temperature.