1 small onion and 1 small carrot, peeled and finely chopped
3cupswhole milk, scalded and simmering
½cup freshly grated Parmesan cheese
½cupGruyère or Emmenthaler cheese, grated into narrow threads
First, blanch the spinach. This will preserve the green color and wilt the leaves. Begin by bringing 4quarts of lightly salted water to a full rolling boil. It is best to use an unlined copper saucepan of the type used for candymaking. The bare copper does an excellent job of keeping the spinach green. Enameled or stainless steel surfaces do well also.
While you wait for the water to boil, remove and discard the stems from each leaf and toss the leaves into cold water in a clean sink or large bowl. Spinach is often gritty EVEN WHEN IT LOOKS CLEAN. Rinse the leaves three times, shaking them in each change of water. Make sure no grit comes off them after the third rinse. If it does, rinse them again. Then drain them in a colander, shake them dry, and let them stand until the water boils.
Put the leaves in the boiling water. Keep the heat high so the boiling will resume as soon as possible. Do not cover the pot as this will promote discoloration. From the time the water comes to the boil, the blanching should take only 2 to 3 minutes. As soon as you think the leaves have softened to the point of fork tenderness, dump them into a colander and run cold water over them to stop the cooking. Stir to quicken the cooling. This impedes yellowing and keeps the spinach from taking on what Madame Saint-Ange calls a mauvais goût.
Handful by handful, squeeze out as much water from the spinach leaves as you can and set them on a cutting board. Chop them roughly and then put them on a clean dishtowel. Squeeze out the rest of the water by twisting both ends of the towel. Proceed right away to preparing the finished dish. Waiting will not improve the spinach.
Melt 2tablespoons of the butter in a medium saucepan over medium heat. When the foam subsides, add the spinach and stir for 5 minutes to dry it out some more. Remove from the heat. Toss in 2tablespoons of the flour, and pinches of salt, pepper, nutmeg, and sugar. Return to the heat and stir for another 2 minutes. Remove from the heat and stir in the cream, gradually. Then set over low heat. Cover and let the spinach cook very slowly for 20 minutes. Do not boil. Remove from the heat and reserve in a bowl.
While the spinach goes through this final cooking stage, prepare the Mornay sauce. Melt 2tablespoons butter in a 6-cup nonstick saucepan. Add the chopped onion and carrot, and sauté over low-medium heat for 10 minutes without browning. The result is a mirepoix, a classic appareil, or prepared instrument, used as an ingredient in many traditional recipes for adding flavor or texture that would otherwise have required an undesirable or impractical amount of cooking if the same ingredients had been added in to the main dish from the start.
Scrape the mirepoix onto a plate. Then, using the same saucepan, without washing it, melt 2tablespoons butter over low-medium heat. When the foam subsides, whisk in the remaining 4tablespoons flour. Continue whisking until the flour and butter have melded smoothly and the flour has cooked without browning, about 3 minutes. This is a white roux, another standard appareil.
Without waiting, pour the hot milk over the roux and whisk vigorously. Remove from the heat as soon as the liquid returns to the boil.
Stir in the mirepoix, then set the sauce over low heat at the edge of the burner so it cooks only in one spot. Cook for 30 minutes, scraping from time to time with a wooden spatula to make sure this thick sauce is not sticking.
Preheat the oven to 450 degrees.
Push the sauce gently through a chinois or other fine strainer into a 4-cup glass measuring cup. You should now have 1½ to 2cups of sauce béchamel, the classic white sauce, which is the base or mother sauce (sauce mère) of a family of more complex sauces, among them Mornay.
Into the hot béchamel, stir ⅓cup each of the cheeses until they melt. Finish with the remaining 2tablespoons butter. Whisk the sauce until smooth.
Spread 4tablespoons of the sauce over the bottom of an oval or round 6-cup ovenproof dish. Then add the spinach. Mound the top with a rubber spatula. Cover the spinach gradually with the rest of the sauce. Sprinkle with the remaining grated Parmesan and Gruyère. Then brush the melted butter on the cheese. Set in the oven for 5 minutes, or until the top melts. If it doesn’t brown, run under the broiler as close as you can get it. Leave the oven door open so you can see what is happening.