Attach multipurpose blade toKitchenAid 7-Cup Food Processor. Add flour, salt and cold butter to work bowl and pulse until mixture forms a coarse texture. Add sour cream and continue to PULSE until dough starts to come together. Transfer mixture to a lightly floured surface gently gather dough to form a disc. Wrap in plastic wrap and refrigerate 30 minutes.
Grease a 23cm x 2 cm round loose bottom tart pan with cooking spray. Remove pastry from refrigerator and let stand for 5 minutes before rolling out. Turn dough onto a lightly floured surface and shape into a flat disc. Flour both sides of disc lightly and roll into a 12-inch circle. Ease dough into prepared tart pan. Trim edges, prick base 12 times and refrigerate for 30 minutes before baking.
Prepare onion filling while pastry is chilling. Exchange multi-purpose blade for slicing disc. Cut onions to fit feed tube and slice on high speed. Heat butter and oil in a large frying pan over medium heat and add sliced onions. Cook, stirring frequently for 20 to 25 minutes or until softened and caramelized. Stir in vinegars, sugar and salt and increase heat to medium high heat. Cook, stirring for 5 minutes or until all the liquid has evaporated and mixture is jammy and thick. Remove from heat and set aside.
Preheat oven and a large flat baking tray to 190C. Cover pastry with parchment paper and fill halfway with baking beads or rice. Place on the hot tray. Bake for 15 minutes. Remove paper and beads and bake for 10 to 15 minutes more or until light golden.
Spoon onion mixture into tart shell and top with thick slices of goat’s cheese. Scatter fresh thyme leaves over top and bake 15 minutes or until cheese is lightly golden. Cool in pan for 5 minutes, serve warm. makes 6 servings