Stir together the turkey, spring onions, tarragon or sage, ham, egg and oats in a bowl. Season well. Form into meatballs, a little bigger than the size of a walnut.
Heat the olive oil over medium heat in a frying pan and fry the meatballs, rolling them around the pan with a spoon and fork to keep them round, until browned on all sides, about 8 minutes.
Stir in the tomatoes and sherry and season with salt and pepper. Bring to the boil, then reduce the heat and cover. Simmer until the meatballs are cooked and the sauce well flavoured, about 20 minutes. Stir occasionally.
The chopped ham is optional, but does add flavour to the meatballs.