Best White Lasagna Recipe - How to Make White Lasagna

Ingredients

• 1 lb. (16 oz.) frozen, chopped spinach, thawed according to package directions
• 2 1/2 tbsp. olive oil
• 8 oz. cremini mushrooms, chopped
• 1 small onion, diced
• 1 1/2 tsp. kosher salt, divided
• 3 cloves garlic, chopped
• 1 lb. whole milk ricotta
• 1 egg
• 1 tbsp. fresh rosemary, chopped
• 1 tbsp. fresh oregano, chopped
• 1/4 c. fresh parsley, chopped
• 6 tbsp. butter
• 7 tbsp. all-purpose flour
• 4 c. whole milk, warmed
• 1/2 tsp. ground nutmeg
• 3 c. (4 1.2 oz.) Parmesan, freshly grated
• 1/2 tsp. kosher salt
• Freshly ground black pepper
• 24 no-boil lasagna noodles (14 oz.)
• 2 c. (12 oz.) cooked chicken, cut into 1/2" pieces
• 3 c. low-moisture whole milk mozzarella
• 1/3 c. Parmesan, freshly grated