In a large skillet, melt the butter in the vegetable oil. Add the sliced onions, cover and cook over moderate heat, stirring, until softened, about 12 minutes. Uncover the skillet and cook, stirring occasionally, until the onions are very soft and well browned, about 45 minutes longer; add a little water as necessary to prevent the onions from sticking. Add the Marsala and cook until nearly evaporated. Season the onions with salt and white pepper and transfer them to a medium bowl.
Meanwhile, preheat the oven to 375°. On a lightly floured work surface, roll out the Buttery Cornmeal Pastry to a 13-inch round, 1/8-inch thick. Wrap the dough around the rolling pin and fit it into a 10-inch fluted tart pan with a removable bottom. Run the rolling pin over the rim to trim off the overhang. Prick the bottom with a fork and refrigerate until chilled, at least 10 minutes.
Line the tart shell with foil and fill with pie weights or dried beans. Bake in the center of the oven for about 25 minutes, or until the edge is golden and the center is nearly set. Remove the foil and weights, cover the edge with foil and bake for 5 minutes longer, until the shell is cooked through and golden.
Sprinkle the Gruyère into the tart shell and spread the onions on top. Bake for 10 minutes, or until the cheese is melted and the onions are sizzling. Let cool for 15 minutes. Unmold the tart, cut it into wedges and serve warm.
The tart can be refrigerated overnight. Rewarm before serving.
This cheesy tart needs a wine that stands up to its intense richness and brightens it a bit. Consider an off-dry Gewürztraminer from Alsace.