Preheat oven to 450 degrees F. To make the peach filling: In a large bowl, stir together sugars, flour, cinnamon and salt. Add peaches and gently toss to coat. Mix together cornstarch slurry and lemon juice and stir into peaches to incorporate. Butter a 12 by 8-inch oval dish and add peach mixture. To make the crumb topping: In a medium bowl, combine dry ingredients, add in butter pieces and combine with hands until crumbly. Top peaches with half of the crumb topping. Place in oven and immediately reduce oven temperature to 350 degrees F. Bake uncovered for 30 minutes, then add remainder of the crumb topping. Reduce heat to 325 degrees F and bake until golden and bubbling, another 45 minutes. Remove from oven and let sit for 15 minutes before serving. Serve with vanilla ice cream or fresh whipped cream.