5 lb. medium peaches (about 12), peeled, sliced 1/2" thick
1/3 cup granulated sugar
2 Tbsp. plus 1 cup all-purpose flour
2 cups old-fashioned oats
1/3 cup (packed) light brown sugar
4 tsp. ground cinnamon
1 tsp. kosher salt
1 cup (2 sticks) chilled unsalted butter, cut into small pieces
2 pints vanilla ice cream
Toss peaches, granulated sugar, and 2 Tbsp. flour in a large bowl to combine. Let sit, tossing occasionally, 30 minutes.
Meanwhile, whisk oats, brown sugar, cinnamon, salt, and remaining 1 cup flour in a large bowl to combine. Using an electric mixer on low speed, gradually add butter. Once it has all been added, increase speed to medium-high and beat until dry ingredients are incorporated (there should be no dry bits left in the bowl), about 3 minutes.
Place a rack in center of oven; preheat to 350°. Scrape peaches and any juices into a 13x9" baking dish. Evenly scatter oat topping over peaches and bake crisp until peaches are soft, fruit juices are bubbling, and topping is a deep golden brown, 60–70 minutes. Transfer baking dish to a wire rack and let crisp cool 1 hour.
Divide among bowls and top with large scoops of ice cream.