1. Slice the avocado in half, remove the pit and peel. Deseed and finely chop the green chilli. Mash the avocado and combine with the lime, crème fraiche, chilli and mustard. Season with salt to taste.
2. Finely chop the tomatoes then peel and finely chop the shallots. Coarsely chop the parsley and coriander. Deseed the red chilli and combine with the other dressing ingredients in a blender.
3. Mix the dressing with the tomatoes.
3. Rinse and chop the lettuce. Peel and slice the red onion.
4. Brush the tuna with olive oil, rub with Mediterranean spice mix and barbecue on high heat for half a minute. Turn the fish over, cover each with a Mature Cheddar Slices and let melt.
5. Toast the buns on both sides then top with the avocado cream, lettuce, red onions and tuna. Serve with salsa fresca.