Slow Cooker Peach Cobbler

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6 ounces dark brown sugar

3 1/2 ounces rolled oats

4 ounces all-purpose flour

1/2 teaspoon baking powder

1/2 teaspoon freshly ground allspice

1/2 teaspoon freshly grated nutmeg

1/4 teaspoon kosher salt

1/4 cup unsalted butter, at room temperature, plus extra for the cooker

20 ounces frozen peach slices