Slice the vegetables, chop the ham. Melt the fat and fry the ham for a few minutes and then, very slowly, the vegetables until they are golden brown. Add the flour and continue frying very slowly till all is a rich brown. Add the stock, herbs and spices and stir till the sauce simmers; simmer for ½ hr. Add the tomato pulp and simmer the sauce for a further ½ hr. Wring the sauce through a tammy cloth or rub it through a fine hair or nylon sieve. Season, add the sherry, if used, and reheat the sauce.