Heat your oven to 350ºF. Arrange the fillo shells on a baking sheet.
Add the walnuts, butter, sugar, cinnamon, vanilla, and salt to a food processor or blender; process until the ingredients come together into a chunky dough. Combine the apples and flour in a medium mixing bowl. Add the walnut mixture; mix well.
Pack about 1 heaping teaspoon of the apple-walnut mixture into each fillo shell. Bake for 10 minutes.
While the baklava bakes, combine the water and honey in a saucepan. Bring to a boil, then simmer for 8-10 minutes or until reduced to a thin syrup. Stir in the cardamom and lemon juice.
Carefully spoon the syrup over the baked baklava, letting it seep into the walnut filling.
Refrigerate until ready to serve.