Heat the oil in a large non-stick skillet over medium-high heat, add the shallots and cook, stirring occasionally, until they start to brown, about 6-8 minutes. Turn heat to medium-low and add 1 teaspoon thyme and a pinch of salt. Continue to cook until onions are soft and deep golden brown, about 20 more minutes. After 18 minutes, stir in the chopped prosciutto.

While the shallots cook, preheat oven to 350°F / 175°C and grease a 24-cm tart pan with removable bottom.

Line the tart pan with the puff pastry. Since the puff pastry is rectangular, I cut a couple of centimeters off each end, use the large piece to line the tart pan and then place the extra strips of puff pastry along the sides of the pan to create more crust. Nothing fancy here. Then prick the base of the tart shell a few times with a fork (this is to prevent the pastry from forming big bubbles while baking).

In a medium bowl, combine the ricotta, egg yolk, grated parmesan, 1/2 teaspoon sea salt, and the remaining 1 teaspoon of thyme. Season with a little pepper and stir well to combine.

Spread the ricotta mixture evenly on the bottom of tart shell. Arrange the onion mixture evenly on top of the ricotta and sprinkle with grated parmesan (about 1 tablespoon). Transfer to the preheated oven and bake until edges of the puff pastry are golden brown, 30 to 40 minutes. Serve warm and enjoy!