Heat oil in a large skillet and fry the onion gently for 2-3 minutes. Add the rice and cook slowly for 5-6 minutes, stirring frequently. Add the vegetable stock, bring to a boil and then simmer for about 15 minutes, or until the rice is tender. Add a little extra water if necessary. Remove from the heat and transfer to a large mixing bowl. Add the carrot, leak, sesame seeds, almonds, cheese, eggs and herbs to the mixture. Mix well and season with salt and pepper. Transfer the mixture to a greased 1 lb loaf pan, levelling the surface. Bake in a preheated oven at 350-F degrees for about 1 hour, until set and firm. Leave in the pan for 10 minutes. To make sauce, melt the butter in a small saucepan and fry the onion until dark golden brown. Add the mushrooms and cook for a further 2 minutes. Stir in flour, cook slowly for 1 minute and then gradually add the stock. Bring to boil, stirring constantly, until thickened and blended. Season to taste. Turn out the nut roast, slice and serve on warmed plates with fresh vegetables, accompanied by the sauce. Garnish with sprigs of Italian parsley if desired.