Heat grill to high. Place lemon juice in a small saucepan and bring to boil on the grates of the grill. Cook until reduced to 1/2 cup. Remove from the grill and let cool slightly. Leave grill on high heat.
Place lemon syrup, habanero, lemon zest, and mayonnaise in a blender and blend until smooth. Season with salt and pepper, to taste, and transfer to a large bowl. Add the onions, celery, and parsley and set aside.
Brush tuna with oil and season with salt and pepper on both sides. Grill for 3 minutes per side or until slightly charred and cooked to medium doneness. Remove from the grill and let rest 5 minutes before cutting into small dice or flaking with a fork. Add the flaked tuna to the mayonnaise mixture and gently mix until combined. Divide the tuna salad among 4 slices of bread. Top the salad with a few sprigs of watercress and top with the remaining bread.