Overnight Merguez and Mushroom Strata

Let 2 tablespoons salted or unsalted butter sit at room temperature until softened.

Meanwhile, if you are using fresh bread, arrange a rack in the middle of the oven and heat the oven to 250ºF (skip this toasting step if you are using stale bread and just cut or tear it). Cut or tear 1 (16- to 18-ounce) loaf levain bread into rough 1-inch pieces (10 to 12 cups). Place in an even layer on a rimmed baking sheet. Bake until just starting to dry out but not browned, about 10 minutes. Set aside to cool.

Coat a 9x13-inch baking dish with half of softened butter, then coat a sheet of aluminum foil with the remaining butter.

Dice 1 small or 1/2 medium yellow onion (about 3/4 cup) and finely chop 3 garlic cloves. Trim and cut 8 ounces cremini mushrooms about 1/2-inch thick (about 2 cups). Thinly slice 4 medium scallions (about 1/2 cup). Grate 8 ounces Gruyère cheese on the large holes of a box grater (about 2 cups). Remove the casings from 12 ounces merguez sausage.

Heat 1 tablespoon olive oil in a large skillet over medium heat until shimmering. Add the sausage, use a wooden spoon to break it up into bite-sized pieces, and cook until cooked through and browned, 4 to 7 minutes.

Transfer the sausage with a slotted spoon to a large bowl. Remove all but 2 tablespoons of grease from the pan. (If there is less than 2 tablespoons grease, make up the difference with olive oil.) Add the onion and garlic to the skillet and cook over medium heat, stirring occasionally, until the onion is starting to soften, about 3 minutes.

Add the mushrooms, season with 1/2 teaspoon of the kosher salt and 1/2 teaspoon of the black pepper, and increase the heat to medium-high. Cook, stirring occasionally, until the liquid the mushrooms release is mostly evaporated and they begin to brown, 6 to 8 minutes. Meanwhile, place 8 large eggs, 1 1/2 cups whole milk, 1 cup heavy cream, 2 tablespoons harissa sauce, the remaining 1 teaspoon kosher salt, and remaining 1/2 teaspoon black pepper in a large bowl and whisk to combine.

When the mushrooms are ready, transfer to the bowl with the sausage. Add the bread, half of the cheese, and half of the scallions, and toss to combine. Transfer to the baking dish and spread into an even layer. Pour the milk mixture evenly over everything, then sprinkle with the remaining cheese. Cover the baking dish tightly with the foil butter-side down and refrigerate for at least 1 hour or up to 24 hours. Cover and refrigerate the remaining scallions.

Arrange a rack in the middle of the oven and heat the oven to 375°F. Let the strata sit at room temperature while the oven heats.

Bake covered for 30 minutes. Uncover and bake until the eggs are set, the top is browned, and the edges are crispy, about 20 minutes more. Sprinkle the reserved scallions on top and let cool for 10 minutes before serving.