Step 1

Preheat the oven to 375° F. Combine the flours and the salt in a medium bowl. Toss the butter in the flour. Use your hands to “shingle” the butter with the flour. Do this by flattening the butter between the palms of your hands. The larger the pieces of butter, the flakier the final dough. The smaller the pieces of butter, the mealier the final dough. For this tart, a flakier crust is better since the toppings are light and won’t weigh down the tart.

Step 2

Finish the dough by adding ice water one tablespoon at a time until the dough begins to come together. When the dough is finished, bring it together into a disc and cover it with plastic wrap and refrigerate for at least 15 to 20 minutes.

Step 3

When the dough is chilled, take it out of the refrigerator and divide it into two pieces. Roll each piece into a ball and roll out on a lightly-floured surface using a rolling pin. Roll the dough into a circle about 1/4-inch thick. Transfer to a baking sheet. Dock (poke holes into) each piece of dough all over the base with a fork. [Editors' note: You can also make one larger tart: Either shape it free-form or transfer it to a tart pan.]

Step 4

Apply egg wash to the edges to the tart shell. Bake in the oven until just starting to brown, about 7 to 8 minutes. Remove from oven and allow to cool.

Step 5

In a small sauté pan, heat the olive oil over medium-high heat. Add the prosciutto and sauté until it begins to become crispy. Add the leeks and garlic and continue to cook until the leeks are soft.

Step 6

Remove from the heat and add the Boursin. Mix to combine, season with red pepper flakes, salt, and pepper. Set aside until needed.

Step 7

Spread the leek mixture across the par-baked crusts or crust. Gently top with the raw egg.

Step 8

Bake the tart in the oven until the egg white is set and the crust is fully browned, about 5 to 6 minutes. If a less runny egg is desired, add an additional 2 to 3 minutes Remove from the oven and serve on warm plates.