Preparation

1. Preheat the oven to 375°F with a rack in the middle position.
2. Insert the chopping blade into the work bowl of the food processor. Add the flour and ½ teaspoon of the salt, and pulse to sift. Add the cubed butter over the flour mixture. Pulse 8 to 10 times until the mixture resembles coarse crumbs. Add 1 tablespoon of the ice water. Pulse to incorporate the water. Add the remaining ice water 1 teaspoon at a time, pulsing after each addition just until the mixture forms a clumpy dough that holds when pinched together. Form the dough into a flat disc. Wrap in plastic wrap and refrigerate for at least 30 minutes before rolling.
3. While the dough is chilling, insert the medium shredding disc assembly, and secure the food processor lid. Shred the cheese, using medium pressure; reserve.
4. Once dough is chilled, roll out to a 12-inch by 8-inch rectangle, about ⅛ inch thick. Cut into nine rectangles of about equal size, and lay on a parchment-lined baking sheet. Using your fingers, form a shallow lip around the crust by folding over the edges on each side of the dough rectangle. Chill for 20 minutes. Once chilled, use a fork to prick the center of the dough evenly, making sure not to press entirely through the dough. Lay parchment paper over the each piece of dough, and weigh each piece down with pie weights, dried beans, or rice so that they do not puff while baking.
5. Bake for 15 to 20 minutes, or until the dough under the parchment is no longer wet. Remove the weights and parchment, divide the shredded Gruyère evenly among the shells, and bake until the shells are dry and lightly golden but not browned, about 10 more. Remove from the oven and reserve.
6. While the shells are baking, heat the olive oil in a medium skillet over medium-high heat. Add the asparagus, remaining ¼ teaspoon salt, and the pepper, and sauté until the asparagus is bright green and crisp-tender. Remove from the heat and reserve.
7. Divide the béchamel evenly among the tartlet shells. Top with the asparagus and sprinkle with salt.
8. Serve immediately.

Directions for Béchamel sauce:

1. Put the butter into a large sauce pan and melt over medium heat. Add the flour and stir with a wooden spoon (this is called "making a roux"). Cook the roux over the heat for about 3 minutes, stirring continuously.
2. Whisk in the milk a little at a time until smooth. Turn heat to medium-high and bring the sauce to a heavy simmer. Reduce heat and continue to stir occasionally, maintaining a slight simmer until sauce thickens and coats the back of a spoon. Remove from heat and season with the salt and nutmeg.