"Here, one shaggy dough does double the work: First, as a oatmeal-cookie like foundation. Second, as a streusely topping. Between is a gooey layer of jam. I prefer a dark-hued variety, like blackberry or blueberry or plum, for color contrast...."
INGREDIENTS
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3 cups rolled oats, divided
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3/4 cup brown sugar
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2 1/2 teaspoons kosher salt
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1 cup unsalted butter, roughly chopped, plus more for greasing
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1 (13-ounce) jar blackberry jam (about 1 heaping cup)