Cut watermelon into manageable pieces and remove as much of the red as possible it is OK if a bit remains.
Remove the green outer part of the rind.
With julienne peeler or mandolin cut rind into the noodles.
Scrape pulp and wash preserved lemon, chop coarsely and put in small food possessor, add parsley, black pepper and chilli flakes. Pulse. Keep going until you like the texture. You will need 1-2 teaspoons ( approximately ) sauce for 1 cup of noodles