The recipe for Lidia's Kitchen onion tart, found on Lidia's Flexitarian Kitchen, includes instructions for making the dough and filling. This dish serves about four to eight people.
For the onion tart dough, cut 1/2 cup of butter and 3 ounces of cream cheese into small pieces. Soften them in a large saucepan over low heat, stirring with a wooden spoon until smooth. Add 1 cup of flour a little at a time, stirring until a soft dough forms. Cover and refrigerate for at least one hour.
For the filling, heat 3 tablespoons of olive oil and 1 tablespoon of butter in a large skillet over medium-high heat. Add 3 large onions cut into petals, sautéing for 10 minutes. Reduce the heat to low, and cook for 30 more minutes, stirring once after 15 minutes.
Lower the heat, and add a little more olive oil. Stir in 1 tablespoon of fresh thyme leaves and a generous splash of brandy or balsamic vinegar. Season with salt and pepper, and cook for 10 more minutes. Preheat the oven to 350 degrees Fahrenheit.
Let the dough come to room temperature while grating 3/4 cup of semi-hard cheese. Roll out the dough on a slightly floured surface, and transfer to a baking sheet. Spread 2/3 of the cheese on the dough, leaving a 1-inch edge all around. Arrange the onions on the cheese, and fold the sides over, leaving the middle open. Spread on the remaining cheese, and bake for about 35 minutes.