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Dry-fry the pumpkin seeds in a frying pan for a few minutes, tossing and turning them until they look toasted. Tip them out of the pan and let them go cold. Peel the papayas, halve them lengthways and scoop out the seeds. Cut the flesh into long, thin slices. Halve the avocados and remove the stones, then peel off the skin and slice the flesh lengthways into thin slices.
Put the papayas, avocados, pumpkin seeds and watercress into a large bowl. Chop about 1 tbsp of the mint leaves and set aside. Pick the remaining leaves from the stalks and tear them into the bowl.
Mix the lime juice and olive oil with the rest of the chopped mint to make a dressing, and season with salt and pepper to taste. Pour over the salad and gently mix all the ingredients together with your hands. Taste and add more salt and pepper if you think it needs it, then transfer to a serving platter.