Preheat the oven to 350. The rack should be in the center of the oven.
Open one pack of the 1 package defrosted filo dough from the box, unroll it (you will be unrolling multiple layers), and place a damp cloth over all of the layers.
Melt the 3 tbsp unsalted butter and 3 tbsp olive oil together (I microwaved it in a small bowl for 20-30 seconds). Note: start with 3 tbsp of each, but know that depending on your pastry brush and how you use it, you may or may not need a bit more to finish the project.
Slice your 4-6 ea apples however, you like, but slice them, don't quarter or chunk them. I love to slice them across, versus along the core, for the "star" shape in the middle 🤩.
Lay your first layer of dough down. Ultimately you will lay a total of 6 layers of the dough sheets, using it all up and spreading oil on each layer, before you lay the next. How you decide to lay the initial layer (and subsequent layers) will depend on the pan/dough size. For example, my dough sheets were pretty small (about 8"x 13") and my sheet pan was large (13" x 18"), so I laid three sheets across the 18" length of the pan, overlapping them 1-2"(see photo).
Begin the layering process. In between each layer, you will brush the dough gingerly with the butter/oil mixture. In an effort to keep this dessert light and avoid any greasiness, please take care with this process. Specifically, I brushed along the 4 edges, and then across the dough every couple of inches. Do not cover the entire surface with the fat mixture. I experimented with drizzling a bit of the 1 tbsp maple syrup over the surface as well. You could also sprinkle some of the 1 tbsp white sugar, or you don't have to add any sweetener at all.
Now that the dough is all layered onto the pan, you can lay the apples across the dough. Leave about 1" of the dough uncovered, around the edges so that you can fold the edges in at the end.
Sprinkle roughly chopped 1/2-1 cup walnuts over the apples if you like, as well as the 1 tsp cinnamon, a small sprinkle of 1 tsp salt (always a nice addition), and if you like, another drizzle of the maple syrup and/or a sprinkle of the white sugar. The sweeter you like things, the more you should use them. (I went very lightly w/the sweeteners, both between the dough layers and over the top, and felt the tart had the perfect level of sweetness, which is to say, not very sweet. The apples are the star here).
Fold up the edges of the tart, making your way around the tart. It's a casual process. Brush a tad more fat over the folded edges for the perfect crispy finish!
Your masterpiece is ready for baking. Place in oven and bake for approximately 40 min. or until nicely browned. I used a pizza cutter to cut into squares. You don't have to wait until it cools. Store at room temp, loosely covered. If you reheat it, don't use the microwave. It will just get soggy/chewy.