1) Combine the tuna, Dijon mustard, chipotle, oil, honey and spring onion in a bowl and season with salt and pepper. Shape the chopped tuna firmly into 8 round uniform patties about 4cm thick. Place in the refrigerator for 30 minutes. (The burgers must be very cold to hold their shape when cooking.)
2) Grill burgers for 3 minutes on each side for medium doneness.
3) For the tapenade aioli: Combine olives, capers, garlic, lemon zest and lemon juice in the bowl of a food processor and process until coarsely chopped. Add the mayonnaise and process until smooth, season with salt and pepper. Cover and refrigerate for at least 30 minutes before serving.
4) Serve burgers with ciabatta rolls and aioli. Garnish with watercress.