In a medium bowl, stir together oats, pecans, brown sugar, flour, and salt. Stir in melted butter with a fork until moistened. Freeze until firm, about 20 minutes.
For filling:
In a large bowl, whisk together sugar, flour, and salt; stir in blackberries and lemon juice until combined. Divide mixture among 4 (6-inch) cast-iron skillets. Divide butter among skillets.
Sprinkle topping in large pieces onto filling.
Bake until topping is golden brown and filling is bubbly, about 45 minutes, covering with foil during last 10 minutes of baking to prevent excess browning, if necessary. Let cool for at least 20 minutes. Serve with ice cream.
Recipe by Southern Cast Iron at https://southerncastiron.com/individual-blackberry-crisps/