Place the spring onions (trimmed and finely chopped) in a bowl with the garlic (crushed), ginger, coriander (finely chopped), tuna (drained and flaked), breadcrumbs, egg and lemon juice. Mix the ingredients together.
Season and divide into 4 portions. Roll each into a ball and press flat. Chill the mixture in the fridge for at least 20 minutes or until needed.
Heat a drizzle of oil in a nonstick frying pan and fry the burgers for 2 minutes on each side until golden brown. Squeeze over more lemon juice to taste.
Meanwhile, cut the buns in half and lightly toast them on the cut side. Fill with a burger and a handful of watercress and serve with the remaining watercress and the chopped tomatoes.