In a large bowl combine the walnuts, apples, lemon juice, breadcrumbs, brown sugar, ground cinnamon, salt and 1/2 cup of the melted butter. Stir well to combine and set aside.
Butter the bottom of a 9 x 13 inch rectangular glass baking pan. Place 2 sheets of phyllo on the bottom of pan. Your sheets of phyllo will be too large to line the bottom of the pan perfectly. This is good. Leave one end of the phyllo sheets hanging over the long end of your pan. Using a pastry brush or your fingers, brush on some melted butter. When brushing the butter on the phyllo sheet do so lightly. The goal is not to saturate the phyllo with butter. Then, add 2 more phyllo sheets, this time letting the phyllo overlap on the opposite end of your pan. Brush on more melted butter. Repeat with 2 more phyllo sheets, overlapping this time on the original side. So, to summarize, your bottom layer of apple baklava will be 6 sheets of phyllo, with butter being applied between every second sheet, and with phyllo hanging over the two long sides of your pan.
Onto this bottom layer of phyllo, evenly spread 1 1/2 cup of the apple and nut mixture that you have prepared. Note that you will not have enough filling to completely cover the layer of phyllo; this is fine. Top this with 4 more layers of phyllo, brushing on butter after every second phyllo sheet. Again, remember to allow the excess phyllo to hang over alternate ends of your pan.
Evenly spread 1 1/2 cups of the apple and nut mixture. On top of this add 4 layers of phyllo, as above. Add the rest of the apple and nut mixture, and then top with another 6 to 8 layers of phyllo. When doing so (that is, creating to top of your apple baklava), use your overhanging phyllo sheets. Simply bring them over, two at a time, to cover the apple baklava. You will need to cut off carefully (and discard) some of the excess phyllo dough. Lightly butter the top layer of phyllo.
Using a sharp knife, carefully score the baklava making 3 cuts lengthwise, and then creating a type of herringbone pattern alongside these cuts. This is not an exact science and in fact, the only thing you really need to know is that the vents you will create by scoring the phyllo will help it to bake properly. They will also be your guides when it comes to cutting out serving pieces.
After you have scored your phyllo dough, sprinkle it with about a tablespoon of water using your fingertips; this will prevent the pastry from curling. Place in the top rack of an oven set at 350 degrees Farenheit (177 degrees Celsius). Bake for approximately 30-40 minutes, or until top is golden brown.
Remove from oven, and while your apple baklava is still hot, pour on the cooled syrup. Pour the syrup evenly over all parts of the apple baklava.
Allow to sit at least a couple of hours before cutting and serving.
Apple baklava can be kept at room temperature for several days. Do not cover it tightly with plastic wrap, as this will cause your phyllo to get soggy. Instead, when it has cooled completely, use a clean tea towel or piece of cheesecloth to cover your apple baklava. This will keep it fresh and crispy.