Preheat oven to 400°F (200°C). Grease an 8x8-inch (or comparable-sized) baking dish and set it on a rimmed baking sheet; set both aside.

In a large bowl, whisk together 1 cup flour, ½ cup sugar, 2 teaspoons baking powder, ½ teaspoon salt, and ½ teaspoon cinnamon until evenly combined.

Stir ½ cup milk into the dry ingredients just until the batter comes together; do not overmix.

Gently fold in 3 cups sliced peeled peaches until they are evenly distributed through the batter.

Spread the batter and peaches into the prepared baking dish in an even layer.

Place the baking dish (on the rimmed baking sheet) in the preheated oven and bake 40–50 minutes, or until the top is golden, the center is set, and peach juices are bubbling around the edges.

Remove from the oven and let cool at least 10 minutes before cutting and serving.