Fragrant and sweet browned onions are the foundation of this savory custard tart. It may have rich ingredients, but it won't make you feel heavy. I love how aged or smoked Gouda adds some extra depth of flavor to the tart, but Gruyère or Comte are good substitutes. If you do not have whole wheat pastry flour you can use all whole wheat flour instead. You can make the pastry dough and cook the onions a day ahead. The pastry dough will keep in the refrigerator covered in plastic wrap for 3 days. Or, freeze it for up to 3 months. The onions are best eaten within 24 hours of making them. Onion tart is perfect for brunch, lunch, a light supper or as an appetizer for a cocktail party. It is a very versatile food you can make all year long. Special equipment: 9-inch (23 cm) tart pan with removable bottom. You can use any shape, square, circle or an 11 x 8.5-inch ( 28.5 x 20 cm) rectangle tart pan. Stand mixer (optional) Rimmed sheet pan large enough to hold your tart pan. This recipe is slightly adapted from Vegetable Literacy by Deborah Madison.
If using the prosciutto, add 1 tablespoon of extra virgin olive oil to a skillet and turn the heat to medium-high. Slice the prosciutto into strips, about a 1/2-inch (1 cm) wide and not longer than 2-inches (5 cm) long. When the skillet is hot, add the sliced prosciutto and cook until the strips are brown and crispy. Stir occasionally to prevent the strips from sticking and burning. About 5 minutes. Turn off the heat and spoon the prosciutto slices on a plate. Set aside.
If you are using bacon add the two pieces of bacon to a hot and dry skillet and cook the bacon until they are brown and crisp. Turn the pieces over every now and then for even browning. Remove the bacon from the pan onto a plate lined with paper towels and pat dry. When the bacon is cool, crumble them into bite-size pieces. Set aside.
Remove the tart dough from the refrigerator and let it rest on a lightly floured work surface for 10 minutes. Pound the dough with your rolling pin a few times to relax and shape the dough. Roll the dough in the shape of your tart pan to about a ¼-inch (.5 cm) thickness, and large enough to fit the shape of your tart pan with a slight overhang. For a 9-inch (23 cm) round tart pan the diameter should be around 12-inches (30 cm).