For squash filling

• 1 large onion, chopped
• 3 tablespoons unsalted butter
• 3 lb butternut squash, peeled, seeded, and cut into 1/2-inch pieces
• 1 teaspoon minced garlic
• 1 teaspoon salt
• 1/4 teaspoon white pepper
• 2 tablespoons chopped fresh flat-leaf parsley
• 4 teaspoons chopped fresh sage
• 1 cup hazelnuts (4 oz), toasted, loose skins rubbed off with a kitchen towel, and coarsely chopped

For sauce

• 1 teaspoon minced garlic
• 3 tablespoons unsalted butter
• 5 tablespoons all-purpose flour
• 5 cups milk
• 1 bay leaf (not California)
• 1 teaspoon salt
• 1/8 teaspoon white pepper

• 1/2 lb fresh mozzarella, coarsely grated (2 cups)
• 1 cup finely grated Parmigiano-Reggiano (3 oz)
• 12 (7- by 3 1/2-inch) sheets no-boil lasagne (1/2 lb)