Cheesy Caramelized Onion Tart on Potato Crust

Start caramelizing the onions, since they take the longest. In a heavy, large skillet or pot, heat half of the butter and olive oil over medium-low heat. Add the sliced onions and stir. Sprinkle lightly with salt. Cook for 40 to 60 minutes, stirring occasionally, until the onions are browning and very stringy. During cooking, you should hear a constant, very gentle sizzle. Using too much heat will brown the onions before they’re caramelized, so they’ll be missing the sweet taste. Remove them from the pan and set aside in a bowl when done. Don’t clean the pan because you’ll need it in a bit.

To make the crust (while the onions caramelize), preheat the oven to 400ºF and shred the potato on a box grater or using the grating disc of a food processor. Place in a colander and sprinkle with salt and pepper. Wait a few minutes, then use your (clean) hands to squeeze the potatoes into a ball, pressing out as much liquid as possible.

Grease a standard-sized pie plate (glass preferred) generously with softened butter or olive oil. Sprinkle the grated potato evenly around the pie plate, then press down into a firm layer on the bottom of the pan. Bake for about 40 minutes, until the bottom of the crust is browning and the top appears dry. Remove it from the oven and set aside. Reduce oven temperature to 350ºF and place the oven rack just above the middle position.

For the mushrooms, in the pan used to caramelize the onions, heat the butter and olive oil over medium-high heat. Add sliced mushrooms and stir. Cook, stirring infrequently, until the mushrooms release their juices and then the juices have evaporated entirely (patience). They should be just starting to stick to the pan. Remove from heat and set aside.

For the egg mixture, in a medium bowl, whisk the eggs, half-and-half, and mascarpone until blended. Add the shredded Fontina and salt and pepper (to your preference, or ¼ teaspoon each).

To build the tart (the fun part), use a brush to spread the mustard in a thin layer over the cooled crust. Spread the caramelized onions in an even layer over the crust, then add the cooked mushrooms. Pour the egg mixture into the pan. It should work its way into the nooks and crannies, but if not, tilt the pan slightly to help it along. Arrange the tomato slices gently over the top and sprinkle with salt, pepper and the Parmesan cheese.

Put it into the oven and bake for about 40 minutes, or until the tart is set (it should only jiggle slightly in the middle) and browned on top. If you’re using a different pan size, such as for a double recipe, check the tart earlier and be prepared for a shorter or longer cooking time. Remove from oven.

Allow to cool before serving so the tart can set up more, giving you nicer-looking pieces.