Whisk the flour and salt together in a large bowl. Process the butter with the flour mixture until the butter is the size of large peas, then stir in the sour cream, ice water, and lemon juice until a dough forms. Wrap in plastic and chill for at least 1 hour.

Place the sliced potatoes in a microwave-safe bowl, cover tightly, and microwave until they are just tender when pierced with a knife, 4 to 8 minutes. Drain the potatoes thoroughly and return them to the bowl to stop the steaming process.

Heat the oil in a 12-inch skillet over medium heat until shimmering. Add the shallots and rosemary, cover, and cook until the shallots are softened and beginning to brown, 5 to 7 minutes.

Add the shallot mixture to the potatoes. Gently stir in the crème fraîche, olives, and lemon zest, then season with salt and pepper to taste.

Roll the chilled dough into a 12-inch circle on a piece of parchment paper. Transfer the parchment and dough to a baking sheet.

Spread the potato mixture over the dough, leaving a 1 1/2-inch border around the edge. Sprinkle the crumbled goat cheese evenly over the potatoes.

Fold the edges of the dough over the filling, pleating it every few inches to create a rustic border; the center of the tart should remain open.

Bake in a 400°F oven until the crust is deep golden brown and the cheese has softened, 30 to 35 minutes. Let the galette rest for 10 minutes before slicing to allow the filling to set.