Heat the oven to 375°F and lightly butter a 10-inch cast-iron skillet.
Toss the apples with 1/4 cup brown sugar, lemon juice, 1/2 teaspoon cinnamon, salt and cornstarch.
Spread the apples evenly in the skillet, pressing down any wedges that stand above the rim.
Mix the oats, flour, remaining brown sugar and remaining cinnamon in a bowl.
Rub the cold butter into the oat mixture with your fingertips until moist clumps the size of peas form.
Scatter the topping evenly over the apples without packing it down.
Bake for 38 to 42 minutes until the top is brown and thick apple juices bubble around the edge.
Cool for 20 minutes before spooning the crisp into bowls so the filling can thicken.