Lay the tomato slices on a plate, season with salt and drizzle with the oil and vinegar. Set aside for a few minutes until the tomato juices have been drawn out (reserve this juice on the plate). Mix the soft cheese and pesto together in a bowl.

Toast the bagels, then spread the cut sides with the soft cheese mix. Top with the tomato and avocado slices. Dress the watercress in the tomato juice, then pile on top and sandwich with the bagel tops. Press down lightly and serve.