Sauté onion and sugar over a very low heat in a covered pan for approximately 15 minutes. Then add the wine and cook uncovered for an hour over low heat until it begins to caramelize.
Season tuna with salt and pepper, and rub with a little oil, and grill over open flame for approximately 3 minutes per side or until just cooked through.
Brush with a small amount of oil and grill the slices of bread. Spread with a layer of hot glazed onion. Cover with a slice of ham. Add 2 strips of tuna then sprinkle with a few grains of sea salt, a pinch of chervil, and 2-3 drops of lime juice.
Serve immediately. The toast and the glazed onion should be very hot so that the ham fat melts a little.