Peel the watermelon and cut it into wedges. Use the red pulp in another way. Peel off the whitish-green part of the watermelon skin and cut it into bite-size pieces.
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Lightly toast the fennel seeds in a non-stick pan. Squeeze the lime. Bring both to the boil with the pepper, vinegar and water. Add the thyme, sugar, salt and bring to a simmer briefly. Combine the watermelon pieces and bring to the boil again. Pour into jars rinsed with boiling water, filling them up to the brim. Add the liquid so that it completely covers the watermelon chunks. Immediately close hermetically. In the dark and cool, watermelon pickles will keep for a few weeks.