Parisian Gnocchi

Ingredients:
1 1/4 cups (310ml) water
7 tablespoons (3 1/2 ounces/100g) unsalted butter, room temperature, cubed
1/2 teaspoon sea salt or kosher salt
1 1/4 cups (175g) all-purpose flour
4 large eggs, at room temperature
2 teaspoons dry mustard powder
5 tablespoons (2 1/2 ounces/70g) salted or unsalted butter
1/3 cup (45g) all-purpose flour
3 cups (750ml) whole or low-fat milk, warmed
1 teaspoon sea salt or kosher salt
Generous pinch of cayenne pepper
1 3/4 cups (140g) grated Swiss-style cheese, such as Emmenthal, Gruyère, or Comté
1/3 cup (1 ounce/30g) freshly grated Parmesan cheese

Instructions:
1. To make the pâte à choux, heat the water, butter, and 1/2 teaspoon of salt in a saucepan over medium heat just until the butter is melted. Dump in all the flour at once and stir the mixture briskly for about 2 minutes, until the dough forms a smooth ball. Remove from the heat and scrape the dough into the bowl of a stand mixer fitted with the paddle attachment. (If you don't have a stand mixer, simply leave it in the bowl.) Let the dough sit for 3 minutes, stirring it every so often to release some of the heat.
2. With the mixer on medium-high speed, add the eggs one at a time, making sure each one is fully incorporated before adding the next. Add the dry mustard and beat until the dough is completely smooth. Cover the bowl with a kitchen towel and set aside.
3. To make the mornay sauce, melt the butter in the saucepan over medium heat. Add the flour and cook, letting the mixture bubble, stirring constantly for 2 minutes, until the paste is thickened. (Don't let it brown.) Gradually whisk in the milk, beginning slowly and stirring constantly to avoid lumps.
4. Decrease the heat to low and cook the mornay for 6 minutes, stirring frequently, or until the sauce is about as thick as a milkshake. Remove from heat and add the salt, cayenne, and 1/2 cup (40g) of the Swiss-style grated cheese; stir until the cheese is melted.
5. Butter a shallow 2 1/2- to 3-quart (2.5 to 3l) baking dish. (A wide dish is preferable to a deep one for browning the cheese topping.) Sprinkle half of the Parmesan over the bottom and sides. Spread 1 cup (250ml) of the mornay sauce over the bottom of the baking dish.
6. Line a large dinner plate with a few layers of paper towels. Bring a pot of salted water to a low boil. Either using two soupspoons—one to scoop up some of the dough and the other to scrape it into the boiling water—or a spring-loaded ice cream scoop filled partially full, scoop a round of the dough—about 1 generous tablespoon each—and drop it into the water. (The ice cream scoop was a little newfangled for Paule, although she did agree—reluctantly—that it was more expedient and made nicer gnocchi.) Working in batches, poach 8 to 10 gnocchi at a time. Let them poach for 2 minutes, then retrieve them from the water and drain them on the paper towels. (They won't be fully cooked inside.) Repeat, poaching the rest of the gnocchi the same way.
7. Preheat the oven to 350ºF (180ºC) with the oven rack in the top third of the oven.
8. Once the gnocchi are parcooked, place them in a single layer on top of the mornay in the baking dish, and then spoon the rest of the mornay over the gnocchi in a fairly even layer. Sprinkle the remaining 1 1/4 cups (100g) of Swiss-style cheese over the top, along with the remaining Parmesan. Put the baking dish on a foil-covered baking sheet and bake for 15 minutes. Increase the oven temperature to 400ºF (200ºC) and bake for another 15 to 20 minutes, until the cheese on top is well browned. Let cool a few minutes, and then serve in the baking dish, family style.