6 oz. thinly sliced Jambon de Paris
3 Tbs. unsalted butter; more for the baking dish
3 lb. leeks
1 Tbs. thinly sliced fresh chives
2-1/2 cups whole milk
8 large eggs
2 Tbs. Dijon mustard
1/2 tsp. finely grated lemon zest
8 cups 1-inch-cubed sourdough bread
8 oz. coarsely grated Gruyère, Emmentaler
Position a rack in the center of the oven and heat to 350°F. Butter a 9×13-inch or similar 3-quart baking dish.
Trim the leeks, leaving the white and light-green parts. Slice in half lengthwise, then crosswise into 1/2-inch-thick half-moon slices to yield about 8 cups. Rinse well, then pat dry.
In a large skillet, heat the butter over medium heat. add the leeks, half of the chives, 1/4 tsp. salt, and 1/4 tsp. pepper. Cook, stirring occasionally, until the leeks are very tender but not browned, 8 to 10 minutes. Allow to cool slightly.
Meanwhile, in a large bowl, whisk the milk, eggs, mustard, zest, the remaining chives, 1/2 tsp. salt, and 1/4 tsp. pepper. Add the bread, cheese, ham, and the leek mixture, and toss well. Transfer to the prepared dish and bake until golden brown around the edges and set in the center, 50 minutes to 1 hour. Let rest 10 minutes before serving.
Recipe credits: Mindy Fox