Preheat your oven to 350°F (175°C). In a large mixing bowl, combine sliced peaches, granulated sugar, and lemon juice. Toss everything together gently to coat the fruit evenly.
Pour the peach mixture into a greased baking dish, spreading it out into an even layer. This will be the juicy filling of your crumble.
In another bowl, combine the flour, brown sugar, oats, and cinnamon. Use a pastry cutter or your fingers to cut in the cold butter until the mixture resembles coarse crumbs.
Sprinkle the crumbly topping evenly over the peaches, covering all the fruit with a generous layer of streusel.
Place the baking dish into the preheated oven and bake for about 40-45 minutes, until the topping is golden brown and the filling is bubbling around the edges.
Remove from the oven and let it cool slightly for about 10 minutes. Serve warm, optionally with a scoop of vanilla ice cream or a dollop of whipped cream.