Ham à la Jurasienne

• 1 Spanish onion, coarsely chopped
• 2 carrots, coarsely chopped
• 6 tablespoons butter

Sauté coarsely chopped onion and carrot in a heatproof casserole with 4 tablespoons butter until golden. Sprinkle lightly with flour; moisten with beef stock to which you have added tomato concentrate, a bouquet garni, and a little salt and freshly ground black pepper; simmer gently for 20 minutes. Strain sauce and add port.

Mix freshly grated Gruyère, egg and double cream to a s