1. Preheat oven to 350°.
2. To prepare topping, place first 3 ingredients in a large bowl; beat with a mixer at medium speed until light and fluffy. Add vanilla and egg; beat well. Weigh or lightly spoon 1 cup flour into a dry measuring cup; level with a knife. Combine 1 cup flour, oats, baking powder, and salt; stir with a whisk. Add flour mixture to sugar mixture; beat at low speed until blended. Cover and chill 30 minutes.
3. To prepare filling, combine sliced peeled peaches, 1/3 cup granulated sugar, 2 tablespoons flour, and fresh lemon juice in a bowl; toss to coat. Spoon mixture into a 13 x 9–inch baking dish coated with cooking spray. Dollop 12 mounds of chilled dough over peach mixture at even intervals. Bake at 350° for 40 minutes or until lightly browned and bubbly.