Preheat oven to 350°F and grease a 9x13-inch baking dish.
Toss apple slices with granulated sugar, 1/2 teaspoon cinnamon, and lemon juice, then spread into the prepared dish.
Combine oats, flour, brown sugar, remaining 1/2 teaspoon cinnamon, and salt in a bowl.
Cut in cold butter with your fingers or a pastry cutter until the mixture resembles coarse clumps.
Spread topping evenly over apples.
Bake 40-45 minutes until topping is golden and apple juices are bubbling.
Serve warm with vanilla ice cream or a drizzle of heavy cream.