Tuna is actually more like red meat than white: it can take only a few minutes cooking when grilled or pan-cooked - otherwise it should be braised for a fairly long time (relatively speaking, of course - it obviously won't stand up to being stewed like beef!) This is a recipe for the long-cooked version. Contrary to popular belief, tuna has very lean flesh and pairs remarkably well with bacon or ham, which impart it with moistness and tenderness. Here that role is played by ham, whose flavor also marries well with the flavour of the cooked "salad." All the vegetables have prominent notes of acidity (tomatoes, sorrel) or slight bitterness (lettuce, spinach) that give structure to the dish and provide it with real vivacity despite its fairly long cooking time.
Prepare all the vegetables: blanch, peel and seed the tomatoes; remove the hard central core and cut into eight; trim the stem ends from the lettuce and cut into quarters; carefully clean and wash the sorrel and spinach.
Cut the tuna fillet into 6 equal portions, season with salt (lightly, because the ham is salty) and pepper and wrap with the slices of ham; tie everything in place with a loop of string.
In a large sauté pan, melt the butter and place the tuna packets in the pan, browning them on all sides. Remove to a plate.
Toss the sorrel and spinach in the butter in the pan until wilted, then add the cut-up lettuce and tomatoes. Cook for 20 minutes. Add the tuna packets with any juice that has collected in the plate and the white wine. Cook covered for 1 hour. Season at the end of the cooking time.
Remove the tuna from the pan and remove the string. Place the vegetable garnish in a deep serving dish and place the tuna and ham on top.