Preheat oven to 350 degrees.
To make the crust, in a food processor fitted with a metal blade, grind the walnuts with a few tablespoons of the flour until finely ground. The flour will keep the nuts from turning into paste. Transfer to a medium bowl, add the remaining flour and the baking powder, and whisk to combine thoroughly. In a small bowl, whisk the oil, milk, and a pinch of salt. Add to the flour mixture, stirring until the dry ingredients are completely moistened. Press the crust into an oiled 9-inch tart pan. Placing a piece of plastic wrap between your fingers and the dough will facilitate pressing. Press the last few times with your thumb around the sides where the bottom of the pan meets the side to make sure there is no excess crust there. Use a knife to cut off any excess along the top of the tart pan, then poke holes all over the crust with the tines of a fork. Bake the shell for 5 minutes, then remove from the oven and set aside. Do not turn off the oven.
In a large bowl, combine and whisk butternut squash puree, eggs, maple syrup, milk, vanilla, and salt. Pour into tart shell. Bake about 50 minutes or until set, checking tart after 30 minutes and tenting edges if browning too quickly. Transfer tart to a wire rack to cool for an hour before transferring to refrigerator to cool completely.
Note: For the butternut squash puree, take a medium size butternut squash, cut in half lengthwise and remove seeds. Place on a parchment lined baking sheet flesh side up. Roast in a 400-degree oven for approximately 25-30 minutes, or until the flesh is fork tender. Scoop the creamy roasted flesh out of the skin to use in the filling of the tart.