Cut tomatos in halves, mince the onions in roundels.
On the fire : in a saute pan, melt the butter and brown (dorer en français) both sides of the tuna (2 minutes each side, depending on thickness of tuna pieces) and then take it off the pan.
After taking off the tuna, put the minced onions, tomatos and herbs, add salt and pepper. Keep on fire untils it is brown. Pour the sweet white wine.
In the oven : put the vegetables in you mini glass casserole and put the tuna on top. Close the casserole with its lid.
Cook for 30 mins at 200°C