Butter should be cut into ½-inch cubes and frozen.
Combine flour and salt in food processor with knife blade attachment and process for about 8 to 10 seconds.
Add cold butter cubes and process by pulsing approximately 8 to 10 times or until mixture resembles coarse crumbs. Do not over mix.
Add water through food chute with processor running. Add only enough water to moisten so dough begins to clump. Do not allow the dough to gather into a single mass.
Form into a 4-5 inch disk, wrap in plastic wrap and refrigerate for 30 minutes before rolling. This will make the dough more elastic and easier to handle.
Preheat the oven to 400°F.
Peel the squash, cut into quarters, and remove any seeds. Cut all four pieces into ¼-inch-thick slices.
Place a piece of parchment paper on a baking sheet and combine squash and garlic cloves with olive oil, salt and pepper. Toss to coat evenly and then spread out into a single layer on the baking sheet.
Bake until the squash and garlic are tender, about 20 minutes. Let cool.
While squash is baking, roll out dough on a lightly floured piece of parchment paper, into a 14-inch round. Slide the parchment paper with the dough onto a rimmed baking sheet and refrigerate for 15 minutes before baking.
In a small bowl, stir the sour cream and gruyere together.
Once the squash is done and the garlic is cool enough to handle, peel and add to sour cream mixture. Combine well.
Take the dough out of the fridge and spread the sour cream mixture evenly over the dough, leaving about 1½-inches uncovered on the edges. Arrange the cooked squash on the cream mixture and add the red onion. Fold the edge of the pastry up and over the vegetables to create a folded-over border and an exposed center. Lightly brush the edge of the dough with milk and top with Parmesan cheese.
Bake the galette for 20 to 25 minutes, until the pastry is lightly browned. Let the tart cool slightly on the baking sheet. Cut it into wedges and serve hot, warm, or at room temperature.