Creamy Lamb's-Quarters Gratin

Preheat the oven to 350 ° F with the rack in the middle. Butter a 2-quart shallow baking dish. Bring 1-inch salted water to a boil in a large saucepan. Meanwhile, wash lamb's-quarters in a large bowl of cold water and drain well. Trim any coarse, woody stems at bottom (don't go crazy with trimming; young lamb's-quarter stems cook up tender and delicious). Add lamb's-quarters to pot and cook over medium heat, covered, until leaves are wilted and stems are tender, 2 to 3 minutes. Drain the greens in a large sieve or colander and rinse well under cold running water. Drain again, pressing out excess liquid with the back of a large spoon. Coarsely chop greens and transfer to a bowl. Dry saucepan and reserve. Cook scallions in olive oil with 1/4 teaspoon salt in reserved saucepan over medium heat, stirring, until softened, 3 to 4 minutes. Combine scallion mixture with greens in bowl. Reserve saucepan again. Melt butter in reserved saucepan over medium-low heat and stir in flour. Cook, stirring, 2 minutes, then whisk in milk and bring to a boil, whisking. Reduce heat and simmer sauce, whisking, 2 minutes (it will be thick). Remove from heat and whisk in nutmeg, zest, and salt and pepper to taste. Mix sauce into greens mixture. If the mixture is too thick, thin with a little additional milk; season with salt and pepper. Spread out mixture in baking dish.