Preheat your oven to 375°F (190°C). Lightly grease a 9-inch pie dish or an equivalent 8x8 inch baking dish.

Prepare the peach filling: In a large bowl, gently toss the peach slices with the 1/4 cup granulated sugar, 2 tablespoons of flour, lemon juice, and 1/2 teaspoon of cinnamon until evenly coated. Pour the mixture into the prepared baking dish.

Prepare the crisp topping: In a separate medium bowl, whisk together the 1 cup flour, rolled oats, brown sugar, and 1/2 teaspoon of cinnamon for the topping.

Cut the cold butter into the dry topping mixture using a pastry blender, two knives, or your fingertips until the mixture resembles coarse crumbs the size of small peas. Do not overmix.

Sprinkle the topping mixture evenly over the peach filling, covering the fruit completely. For extra crispness, ensure some butter clumps remain visible on top.

Bake for 35 to 45 minutes, or until the topping is golden brown and the peach filling is bubbly and tender when pierced with a fork. If the topping browns too quickly, loosely cover it with aluminum foil for the last 10 minutes.

Remove from the oven and let cool on a wire rack for at least 15-20 minutes before serving. Serve warm, optionally topped with vanilla ice cream or whipped cream.