Feel free to experiment with the ingredients! You could make any number of yummy fillings.

For the pastry:
1 cup all-purpose flour
1 cup spelt flour
1 teaspoon kosher salt
1 cup (2 sticks) cold unsalted butter, cut into small cubes
1 large egg
1 tablespoon apple cider vinegar
2 to 4 tablespoons ice water
1 large egg, whisked, to use for egg wash assembly

For the filling:
1/2 cup + 1 tablespoon ricotta cheese
1 large egg, beaten
1/4 teaspoon kosher salt
1 tablespoon snipped fresh chives (or 1 teaspoon dried chives)
3 tablespoons minced parsley
1 tablespoon extra-virgin olive oil
1 small leek, thinly sliced, rinsed well, white and light green parts only (about 3/4 cups)
4 ounces ham or prosciutto, thinly sliced

1 cup all-purpose flour 1 cup spelt flour 1 teaspoon kosher salt 1 cup (2 sticks) cold unsalted butter, cut into small cubes 1 large egg 1 tablespoon apple cider vinegar 2 to 4 tablespoons ice water 1 large egg, whisked, to use for egg wash assembly 1/2 cup + 1 tablespoon ricotta cheese 1 large egg, beaten 1/4 teaspoon kosher salt 1 tablespoon snipped fresh chives (or 1 teaspoon dried chives) 3 tablespoons minced parsley 1 tablespoon extra-virgin olive oil 1 small leek, thinly sliced, rinsed well, white and light green parts only (about 3/4 cups) 4 ounces ham or prosciutto, thinly sliced

Make the dough: Combine the flours and salt in the bowl of a food processor. Pulse quickly to combine. Add the cubed butter and pulse until only pea-sized lumps of butter remain.

In a small separate bowl, whisk together the egg and apple cider vinegar. Add them slowly to the bowl of the food processor, pulsing to combine. The mixture will still be quite dry; add the ice water slowly as you continue pulsing until the mixture holds together when you hold a chunk and squeeze it between your fingers.

Divide the dough in half and flatten each half into a disk. Wrap in plastic wrap, and refrigerate for 1 hour or up to 2 days.

When ready to shape the tarts, remove the dough from the refrigerator and roll out each piece on a lightly floured work surface into a rectangle about 1/8-inch thick; trim the dough so it's an even 9x12 inches. Repeat with the second disk of dough.

Cut each piece of dough into thirds so you form nine 3x4-inch rectangles. Lay nine of the rectangles down on a greased or parchment-lined baking sheet. Refrigerate all pieces of dough until ready to assemble the tarts.
Make the filling and assemble the tarts: To make the filling, whisk together the ricotta, egg, salt, chives, and parsley.

In a small saucepan over medium heat, warm the olive oil and add the sliced leek. Cook until soft and fragrant, 7 to 9 minutes. Set aside.

Mix the leeks into the herbed ricotta mixture (alternatively, keep them separate and scatter a few leeks on top of the ham). Spread a heaping teaspoon of leek-ricotta mixture on top of one of the dough rectangles and lay a slice of ham on top of that.

Brush the edges of the tart with beaten egg, and place a second rectangle of dough on top of the first. Use your fingertips to press firmly around the edges, sealing the dough. Next, use the tines of a fork to press gently down around the edges, further sealing the pastry. Repeat with remaining tarts.

Brush the tops of each tart with beaten egg and prick a few times with a fork. Freeze for 30 minutes to 1 hour before baking. During this time, preheat the oven to 375°F.

Bake for 25 to 28 minutes, or until tarts are light golden-brown. Cool for at least 20 minutes before enjoying. Serve slightly warm or room temperature.

Recipe Notes
If you can't find spelt flour or would just prefer not to use it, whole-wheat flour works great, as does swapping in all-purpose and using 100% of that instead.
The apple cider vinegar helps make the pastry nice and tender. It's inexpensive and should be easy to find. Don't worry: The flavor won't come through in the final result.
I find these tarts cook up much more successfully if they're frozen first. If you can't find the time for this step, they'll certainly still work — they'll just spread a little during baking and won't hold their shape quite as well.