Combine chopped peaches, 1½ tablespoons brown sugar, 1 tablespoon lemon juice and ¼ teaspoon salt in a small saucepan. Cook over medium heat, stirring occasionally, until starting to gently simmer. Reduce heat to medium-low and continue to cook, stirring occasionally, until the peaches have softened and released juices, about 12 minutes. Transfer to a small heatproof bowl and let cool to room temperature, about 30 minutes.
Meanwhile, preheat oven to 375°F. Lightly coat an 8-inch-square baking pan with cooking spray. Whisk ⅔ cup milk, 2 eggs, 1 tablespoon chia seeds, 1 tablespoon vanilla, ½ teaspoon each baking powder, cinnamon and cardamom, and the remaining 4½ tablespoons brown sugar and ½ teaspoon salt in a large bowl until combined.
Fold 2½ cups oats into the milk mixture until combined. Spoon about half (about 1 cup) of the oat mixture into the prepared pan; spread in an even layer. Spoon 1 cup of the peach mixture over the oat mixture; spread the remaining oat mixture over the peaches. Top with the remaining ½ cup peach mixture.
Bake until just set in the center and golden brown on top, 25 to 30 minutes. Let cool in the pan on a wire rack for 5 minutes before slicing. Recipe developed by Marianne Williams
