Overnight Merguez and Mushroom Strata

Name: Overnight Merguez and Mushroom Strata
Ingredients: 2 tablespoons salted or unsalted butter; 1 (16- to 18-ounce) loaf unsliced levain or sourdough bread; 1 small or 1/2 medium yellow onion; 3 cloves garlic; 8 ounces cremini mushrooms; 4 medium scallions; 8 ounces Gruyère cheese; 12 ounces uncooked merguez sausage; 1 tablespoon olive oil, plus more as needed; 1 1/2 teaspoons kosher salt, divided; 1 teaspoon freshly ground black pepper, divided; 8 large eggs; 1 1/2 cups whole milk; 1 cup heavy cream; 2 tablespoons harissa sauce, such as Mina