Lovely, year-round onions. How I love them. Here, three types of onion are cooked to varying degrees: sweet caramelized onions and softened leeks sink into a creamy layer of ricotta, tempered by the zing of raw scallion with lemon zest, and bite of black pepper.
I dot the top of this tart with roasted butternut squash cubes (ie leftovers), which lends some heft to each slice. —MeghanVK
Start the pastry: combine the butter and flour in a bowl, and put in the freezer. In another bowl, combine the egg yolk, water, and salt, and place in the refrigerator. Allow both to chill for ten minutes.
Start the filling: heat the oil in a saute pan over medium-low heat and caramelize the onions for ten to fifteen minutes, stirring so they don't burn. Once the onions are mostly brown, add the leeks, and cook for five minutes more, until softened. Stir the leeks and onions into the ricotta, heavy cream, salt, pepper, lemon zest, and scallions. Set aside.
Back to the pastry! Once the ingredients have chilled, dump the quasi-frozen flour and butter into the food processor. Pulse until it forms the texture of clumpy sand; then add the eggs, water, and salt. Pulse again until the dough is just shy of a ball. Tip onto a board and pat into a disc. Allow to rest in the fridge for 30 minutes.
Roll out the dough between two pieces of plastic and press into a 9-inch tart pan. Spread the filling over the crust and tuck in the cubes of squash. Bake at 350 for 35 minutes and allow to rest at room temperature for another 10. Cut into slices and serve.