Serves: Makes 8 to 12 servings
- 2 tablespoons vegetable oil
- 2 tablespoons unsalted butter
- 4 large yellow onions, thinly sliced
- 2 cloves garlic, pressed or minced
- 1 tablespoon sugar
- Pinch of cayenne pepper
- Salt and freshly ground black pepper
- 1 egg
- 2 tablespoons Dijon or green peppercorn mustard
- 1⁄2 cup grated Gruyère cheese
- 1⁄2 cup grated Appenzeller cheese
- 24 Niçoise olives or pitted dry-cured olives
- 6 anchovy fillets, halved lengthwise (optional)
- 1⁄2 cup unsalted butter
- 1 cup all-purpose flour
- 1⁄4 teaspoon salt
- 3 to 4 tablespoons cold water
- For the pastry dough, cut the butter into small cubes and freeze for 5 minutes. In the bowl of a food processor, combine the butter, flour, and salt. Process for 10 to 15 seconds, until the butter is finely chopped. Add 3 tablespoons water and pulse for another 10 seconds, or until the mixture looks like cornmeal. Pinch some of the dough in your fingers; it should hold together, not feel crumbly or dusty. If necessary, add more water, a teaspoon at a time, pulsing once or twice after each addition.
- Turn the dough out onto a clean work surface. Gather about one-quarter of the dough into a small pile and use the heel of your hand to push the dough away from you 2 or 3 times, which helps thoroughly blend the butter and flour, producing a flakier dough. Don't overwork the dough or it will become tough. Repeat with the remaining dough, then use a pastry scraper to gather the dough, forming it into a disk about 6 inches across. Wrap the dough in plastic wrap and refrigerate for at least 30 minutes.
- Preheat the oven to 425°F.
- Heat the vegetable oil and butter in a large skillet or sauté pan over medium heat. Add the onions and garlic and cook for about 20 minutes, stirring occasionally, until tender. Stir in the sugar and cayenne and cook for 10 to 15 minutes longer, stirring occasionally, until the onions are very tender and take on a deep caramel color. Season to taste with salt and pepper. Whisk together the egg and mustard in a small bowl; set aside.
- Roll the pastry dough out on a lightly floured work surface to a 12-inch circle about 1⁄8 inch thick. Use the dough to line a 10-inch tart pan with a removable base. Brush the bottom and side of the tart shell with the egg/ mustard mixture. Cover the bottom of the tart pan with the grated cheeses, top with the caramelized onions in an even layer, and scatter the olives over. Arrange the anchovy halves lengthwise over the tart.
- Set the tart pan on a baking sheet and bake for about 30 minutes, until the pastry is nicely browned. Let cool on a wire rack for 10 to 15 minutes, then remove the sides and cut the tart into wedges to serve warm.