Position an oven rack to the center of the oven and another in the top third of the oven (about 6-inches from the broiler) and preheat to 375 degrees F. Lightly butter 6 6-ounce ramekins.
Stir together the oats, flour, brown sugar, walnuts and salt in a medium bowl. Work the butter into the oat mixture with your fingertips until it is in even pea-size pieces. Evenly divide the apples among the ramekins and top with the oat mixture.
Transfer the ramekins to a baking sheet and bake on the middle oven rack until the fruit bubbles around the sides and the tops are golden, 30 to 35 minutes. Remove from the oven.
Turn the oven to broil, put the baking sheet with the ramekins on the top oven rack and broil until golden brown, about 2 minutes. Let cool about 15 minutes before serving. Serve warm with whipped cream or ice cream.
When measuring flour, we spoon it into a dry measuring cup and level off the excess. (Scooping directly from the bag compacts the flour, resulting in dry baked goods.)
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