To give us a firm, solid layer to hold up to the creamy filling, we started with a mass of creamed butter and rich brown sugar, into which we mixed in oats, flour, chopped walnuts, baking soda, cinnamon and just a touch of allspice for a rounded aroma. I used quick-cooking oats as I had them on hand, but you could use the thicker, old-fashioned rolled oats if you like - you may just want to blitz them once or twice in a food processor or blender to break them down a bit. Walnuts can just be tossed in naked and chopped, but do yourself a favor - toast the nutty nuggets for a few minutes. Toasting pulls the natural oils to the surface and intensifies their flavor while also crisping them up for an increased crunch.
We took about half of the crumbly mixture and placed it in the baking pan, pressing it down to create the crust. Before the filling can go on though, we needed to give the crust a head start, resulting in a golden color and building the structure of the bottom. You'll be wanting to put the filling on as soon as the crust comes out from the oven, but you may be surprised with just how simple it is. The filling is composed of 5 ingredients - sour cream, granulated sugar (use vanilla sugar if you have some stewing in the pantry!), a couple spoonfuls of flour, an egg and two shredded apples. You can peel them if you must, but you really don't need to bother wasting time with that - we left them on because the flecks of color was nice and the peel would soften enough that it would not become obtrusive texturally.
As we only used half of the oat crumble on the bottom, after we scooped the filling onto the hot base, we sprinkled the rest of the fragrant crust mixture over the top to create these layered bars. Since you can't really use a toothpick to check for doneness, the best way to tell when to take them out is by sight - the top should be evenly golden and the center should feel and look set.
Sandwiched between two oat-y cookie layers, the filling of these bars is tangy, with a gentle sweetness, which we found to be somewhat akin to soft cheesecake. Because of those toasted nuts and warm spices in the crust, the mixture comes across more than just a buttery encasement - it had depth and crunch, yet wasn't tough or overpowering - meaning it did its job without overs-shadowing the more delicate apple filling!
Sour Cream Apple Bars