PREHEAT THE OVEN: Set the oven to 350°F (175°C) and lightly grease a 9x13-inch baking dish with butter or non-stick spray.
PREPARE THE PEACHES: If using canned peaches, drain them well and spread evenly in the baking dish. If using fresh peaches, toss them with sugar and lemon juice, then add to the dish. If fresh peaches are underripe, sauté them for 5 minutes with the sugar and lemon juice to soften slightly.
ADD FLAVOR: Drizzle the bourbon over the peaches. Sprinkle the brown sugar, cinnamon, and nutmeg evenly on top. Gently stir the peach layer to distribute the flavors without breaking up the fruit too much.
ADD THE CAKE MIX: Sprinkle the dry cake mix evenly over the peach mixture. Do not stir; it should remain as a dry layer on top.
TOP WITH BUTTER: Lay the thin butter slices evenly over the surface of the cake mix, covering as much of the top as possible to help the cake bake evenly and brown well.
FINISH WITH TOPPINGS: Sprinkle the salt over the top. Add chopped pecans or walnuts if using.
BAKE: Place the baking dish in the oven and bake for 45–50 minutes, until the top is golden brown and the peach filling is bubbling at the edges.
COOL BEFORE SERVING: Let the dump cake cool for 10–15 minutes to allow the filling to set slightly before serving.