Prepare your campfire or camp stove for cooking. You'll need a bed of hot coals for a Dutch oven, or a steady medium heat for a stovetop.
In a large bowl, combine sliced peaches, 1/3 cup sugar, lemon juice, cinnamon, and cornstarch. Stir gently and set aside.
In a separate bowl, whisk together flour, 1/2 cup sugar, baking powder, and salt. Cut in cold butter using a fork or your fingers until the mixture resembles coarse crumbs.
Pour in milk and stir just until a soft dough forms. Do not overmix.
Lightly grease a 10-inch Dutch oven or heavy pot. Pour the peach mixture into the pot. Drop spoonfuls of dough over the peaches, spreading slightly.
Cover with the lid. For a Dutch oven, place 6-8 hot coals underneath and 12-15 on top. For a stovetop, cook on low-medium heat.
Cook for 30-35 minutes, rotating the pot a quarter turn every 10 minutes for even heat. The topping should be golden and a knife inserted comes out clean.
Remove from heat and let rest 5 minutes before serving. Enjoy warm, optionally with a scoop of vanilla ice cream or a dollop of yogurt.