Preheat he oven to 350° F. Prepare the pie dough. While the pastry is chilling, heat the oil in a skillet and add the onions. Slowly cook the onions, stirring from time to time over low heat, until wilted and brown, about 20 minutes. Reserve and let cool.

On a floured surface, roll out the dough into a 10-inch circle. Carefully lift the pie dough and place on a baking sheet. Evenly spread the mustard over the surface of the pie dough, leaving a 1-inch band around the circumference of the dough. Evenly spread the onions over the mustard. Arrange the tomatoes in a spiral, covering the caramelized onions. Lightly season the tomatoes with salt and pepper.

Bake the tart for 25 minutes, or until the edges of the crust are crisp. Remove the tart from the oven, brush with melted butter, and sprinkle with the parsley. Serve warm.