CRUST AND TOPPING:
Preheat the oven to 350°. Lightly butter an 8-inch square baking pan and set aside. Put the flour, oats, brown sugar, cinnamon, and salt into a food processor and pulse to mix. Scatter the butter over the dry mixture and pulse until the mixture resembles a coarse meal and holds together when pressed between your fingers. Press a little more than half of the mixture into the prepared pan, banking it about ½ inch up the sides. Refrigerate for 10 minutes, along with the remaining crust/topping mixture. Remove the crust from the fridge and bake on the center oven rack for 10 minutes. Cool completely on a rack.
Peel and core the apples. Slice, then cut into small and medium chunks. Melt the butter in a medium skillet. Stir in the apples and sauté over moderate heat for 2 minutes, or until soft. Stir in the brown sugar, raisins, and lemon juice and heat for 1 minute. Remove the pan from the heat and stir in the flour. Transfer the apples to a plate and cool. Spread the cooled filling over the crust. Sprinkle the reserved crumble topping over the apples and press it down gently with your fingertips. Bake for 40 minutes. Serve warm, for a pie-like dessert. Otherwise, cool the bars completely in the pan on a rack. Refrigerate the bars for 30 minutes before slicing.