Pasjoli's Ultimate Grilled Cheese Sandwich (Croque Matthieu) Recipe - Los Angeles Times

Ingredients

• 2 tablespoons unsalted butter
• 2 tablespoons all-purpose flour
• 3/4 cup whole milk
• 1 1/4 cups heavy cream
• 1 1/2 cups grated Gruyère cheese, divided
• 2 tablespoons grapeseed oil
• 5 large Spanish onions, sliced (stem to blossom) into 1/4-inch strips
• 1/2 cup unsalted butter, divided
• 1/2 cup Madeira plus more as needed
• 1/4 cup heavy cream
• Salt
• Pedro Ximenez sherry vinegar or any sherry vinegar
• 1 baguette (such as Bub & Grandma's, about 20 inches long)
• 2 ounces prosciutto, divided
• 1/4 cup caramelized onions (see recipe above)
• 2 tablespoons Mornay sauce (see recipe above)
• 1 cup grated Gruyère
• 1 tablespoon grapeseed oil
• 1/4 cup unsalted butter
• Finely cut chives for garnish