Prepare charcoal briquettes. Place three small rocks or other heatproof items in the bottom of a Dutch oven.
Peach Base
Combine the peaches, brown sugar, lemon juice, and flour in a bowl.
Crisp Topping
In a separate bowl, combine the remaining brown sugar, flour, oats, and salt.
Rub in the butter with your fingertips to form coarse crumbs.
To Assemble
Place the prepared peach base in the pie pan.
Sprinkle crisp topping oat mixture over the fruit.
Place the pie pan in the preheated Dutch oven and put on the lid.
Place nine briquettes under the Dutch oven and 18 briquettes on top, in a checkerboard fashion.
Cook until the crisp is golden brown and the filling is bubbling, 40 to 50 minutes. Rotate the Dutch oven halfway through baking, and replenish the coals as needed.