Preheat your oven to 350°F (175°C). In a large mixing bowl, gently toss the sliced peaches with sugar, lemon juice, and cinnamon until evenly coated. Spread the peach mixture evenly in a baking dish.
In a separate bowl, combine oats, flour, brown sugar, and salt. Use a pastry cutter or fork to cut in the cold butter until the mixture resembles coarse crumbs. This creates the crunchy topping.
Sprinkle the crumbly topping evenly over the peach filling, covering it completely for a thick layer of streusel.
Bake in the preheated oven for about 35 minutes, until the topping is golden brown and the filling is bubbling around the edges.
Remove from the oven and let cool slightly. Serve warm, ideally with a scoop of vanilla ice cream for added comfort and flavor.