Scallions come in many shapes and sizes, from tiny and red to thick and green and very, very long. A stack at the St. Paul market labeled "table onions" were nearly as long as King Alfred leeks and just as sweet. Potatoes are, of course, delicious with leeks, so you can expect they'd be good with scallions, and they're a perfect vehicle for herbs, whether bracing parsley, a handful of chervil, tarragon, or whatever herb you love.
It's completely optional, but I often add a little cheese—goat cheese or sheep's milk feta for a bit of tang, Gruyère for a richer version. Leftovers are delicious browned in clarified butter or olive oil. Measurements are flexible.
1. Preheat the oven to 375 F. Lightly butter a 2-quart gratin dish. Chop the leeks into 1/2 inch pieces and wash them well in plenty of water, separating the rings. Let them soak while you trim and quarter the fennel. Slice it very thinly, including the core. Bring a skillet of water to a boil, add the fennel and a pinch of salt. Simmer until the fennel is translucent, about 2 minutes, then drain.
2. Melt the butter in a wide skillet. Lift the leeks out of their soaking water and add them to the pan along with the fennel. Season with 1 teaspoon salt and cook over medium heat, stirring frequently, until the leeks are tender, about 10 minutes, adding the scallions after 5 minutes. Add the fennel greens and lemon zest, taste for salt, and season with pepper. Scrape the vegetables into the prepared dish.
3. Beat the eggs and milk together and add 1/2 teaspoon salt plus the cheese. Pour it over the vegetables, then bake until the top is browned here and there in places, about 40 minutes. Let rest for a few minutes, then serve.
Variation with Endive: Slice or chop 1 or 2 white Belgian endives and cook them with the leeks and fennel.
Adapted from Local Flavors: Cooking and Eating from America's Farmers' Markets by Deborah Madison (Broadway Books, 2002). Copyright 2002 by Deborah Madison.
Andrea Reusing is the creator of the restaurant Lantern in Chapel Hill, N.C., and author of the book Cooking in the Moment: A Year of Seasonal Recipes. In this installment of The Key 3, she shares with Lynne Rossetto Kasper the techniques behind three of her favorite recipes: Turnip Soup, Overnight Braised Short Ribs and Tomato Salad.