Heat a large cast-iron skillet over high heat until smoking. Add the tomatoes to the dry pan and cook, turning a few times, until charred on all sides but still holding their shape, 3 to 4 minutes. Add the onions and then immediately pour in the vinegar. Stir in the confectioners' sugar until dissolved. Cook, stirring often, until the onions soften and the vinegar thickens, 3 to 5 minutes. (The onions should retain some bite; the vinegar should be reduced and be slightly syrupy.) Pour into a serving bowl, making sure to scrape out all the contents of the skillet. Season with the flaky sea salt and drizzle olive oil over the top. Garnish with the basil.
Recipe courtesy of Valerie Bertinelli