For the reduction: In a small pot on the stove, combine vinegar and sugar. Bring to a boil and allow to cook until you have boiled off 1/4 - 1/3 of the mixture. Set aside and let cool to room temperature. For the tart: Preheat the oven to 375 degrees Fahrenheit. Using a cutter that is about 3 1/2" in diameter, make an indentation on each puff pastry circle about 1/2" from the edge. DO NOT cut all the way through the dough. Heat the olive oil in a large frying pan over medium-high heat and add the onion and garlic. Saute until onion is translucent. Add tomatoes and cook until they are heated through. Stir in the basil. Season to taste with salt and pepper. Arrange filling on puff dough circles, being sure to keep inside the indentation you made. Crumble goat cheese over the filling. Drizzle each tart with balsamic reduction. Bake for about 20 minutes, or until the edges of the dough have risen, the dough is cooked through and the cheese is starting to turn a golden colour around the edges. Yield: 4-6 servings (depending on the size of the puff pastry sheet you use).