Creamy Mushroom Lasagna

Name: Creamy Mushroom Lasagna
Ingredients: 1 lb. dried lasagna; 3½ tsp. Diamond Crystal or 2 tsp. Morton kosher salt, divided, plus more; 16 Tbsp. (2 sticks) unsalted butter, divided; 2 lb. crimini mushrooms, trimmed, thinly sliced; 1½ tsp. freshly ground pepper, divided, plus more; 3 shallots, finely chopped; 8 garlic cloves, finely chopped; 2 Tbsp. finely chopped thyme; ½ cup all-purpose flour; 5 cups whole milk; 2 Tbsp. Dijon mustard; 2 tsp. finely grated lemon zest; 1 large egg; 6 oz. Parmesan, finely grated (about 3 cups); 2 cups whole-milk fresh ricotta; 8 oz. low-moisture mozzarella, coarsely grated (1⅓ cups)