Peel the papaya, de-seed and slice thinly. Add to a medium bowl and toss with the lemon juice and sugar to combine. Cover with plastic wrap and leave to sit over night in refrigerator.
The following day, add the mixture to a mortar and pound to a paste or purée in a food processor. Transfer to a medium saucepan, bring to a boil then reduce to a simmer. Continue cooking until all the moisture has evaporated, then spoon into hot, sterilized canning jars and seal with lids and rings.
Allow to cool to room temperature, and refrigerate any jars that do not seal. Store in a cool, dark cupboard.