Preheat your oven to 350°F (175°C). In a large bowl, gently toss the sliced peaches with granulated sugar and lemon juice until evenly coated. Transfer the mixture into a baking dish, spreading it out into an even layer.
In a separate bowl, combine rolled oats, all-purpose flour, brown sugar, and cinnamon if using. Mix well to distribute the ingredients evenly.
Add the chilled, diced butter to the dry ingredients. Using your fingertips or a pastry cutter, work the butter into the mixture until it resembles coarse crumbs, with some pea-sized pieces for texture.
Sprinkle the crumble topping evenly over the peach filling in the baking dish. Gently press down some parts of the topping to ensure good contact with the fruit and even baking.
Bake in the preheated oven for about 35 minutes, or until the topping is golden brown and the filling is bubbling around the edges. You may want to check at the 30-minute mark to prevent over-browning.
Remove from the oven and let cool slightly for about 10 minutes. The crumble will thicken and become even more fragrant as it cools. Serve warm, ideally with a scoop of vanilla ice cream or a dollop of whipped cream.
