Add flour and salt to a food processor, pulsing several times to combine. Add the butter and pulse on and off until the mixture is crumbly.
Slowly add the water with the machine running until the dough begins to form a ball (see notes).
Wrap in plastic wrap and press flat into a disc. Chill for at least 30 minutes before using so the butter can firm back up.
Prepare the Filling
Preheat the oven to 425 degrees F. Line a sheet pan with aluminum foil.
Toss the butternut squash in a very light coating of olive oil and a sprinkle of salt. Roast for 30 minutes, flipping halfway through, until tender. (It will go back in the oven, so it's ok if it's slightly underdone).
While the squash is roasting, heat a medium-sized skillet over medium heat. Add a thin coating of olive oil to the bottom of the pan, about 1/2 tablespoon. Add the shallots and cook for 2-3 minutes, or until lightly caramelized, stirring and adjusting the heat if needed. Add the pecans and cook for an additional minute, followed by the cinnamon, nutmeg, and 1 1/2 teaspoons of dried sage. Continue cooking until the spices are fragrant, then remove from the heat and set aside.
Once the squash has finished roasting, allow to cool for 5 minutes, and then gently toss with the shallot pecan filling (you can use the roasting pan or a large bowl). Lower the oven temperature to 400 degrees F.
Roll the dough onto a lightly floured surface until approximately 1/2 inch thick. Use a pizza cutter to make a circle with the dough that's about 12 inches in diameter. Carefully transfer the rolled dough to a baking sheet covered in parchment paper. (The best way to do this is to gently roll the dough around the rolling pin, then unroll it on top of the baking sheet).
Spread the roasted butternut squash evenly onto the dough, leaving about 2 inches around the edges. Top evenly with goat cheese, the remaining sage, and a light sprinkle of nutmeg.
Fold the excess dough on top of the ingredients in a circle, until you have a tightly formed tart.
Chill for 15 minutes (this helps the dough set and cook evenly).
Bake for 45 minutes, or until the goat cheese is browning and the dough is firm. Allow to cool slightly before cutting.
When using a food processor to make dough, you want the machine running for the least amount of time possible once you add the water. This will help keep the dough flaky. You can let the dough form a ball, but it will be a tougher crust. If you stop the machine once the ingredients are combined and flaky, you can press them together by hand for more tender results. I press them together while wrapping the dough in plastic wrap.