Remove the seeds from the melon and cut into slices. Cut the flesh from the skin. Peel the cucumber, cut it in half lengthwise and scrape out the seeds with a tablespoon. Cut off a piece (about 10 cm long). Use a peeler to cut 4 thin slices from each of the melon and cucumber pieces. Cut the remaining melon and cucumber into large pieces. Wash the lemon balm and remove the leaves from the stalks.
Clean and wash the spring onions and cut the white and light green of the onions into rings. Finely mash the remaining spring onion green, melon, cucumber, lemon balm and buttermilk. Season with salt and pepper. Chill the soup for at least 1 hour. Rinse shrimps in a sieve and drain well. Serve the soup with shrimps, spring onion rings and one slice of melon and one slice of cucumber.
1 hour waiting time