Give your passion fruit a quick wash in the sink, or in a large bowl full of water. Slice using a small sharp serrated knife and use a spoon to scrape the pulp and seeds into a jar, making a note of the weight. Keep the jar in the fridge until needed and reserve the shells.
Add the shells of the passion fruit to a large pot, cover with water and bring to the boil. Simmer for 30 minutes then strain, reserving the cooking water.
Scoop out the tender inner flesh discarding the outer skin. Use an immersion blender or food processor to blitz to a smooth purée. (Remember to use yellow passion fruit if possible).
Combine this purée with the passion fruit pulp you have in your fridge, the sugar, water and lemon juice in a large stainless steel pot. Stir over low heat to dissolve the sugar then increase heat and bring to the boil.
Reduce the heat to low and continue to simmer the jam, only occasionally stirring to make sure the fruit doesn’t catch. Add a small knob of butter to reduce any grayish bubbles (scum) forming.
Cook for approximately 15 minutes, or until the bubbles die down and the jam is glossy. Spoon a small amount onto a cold plate and run a finger through it – it should leave a clean trail. Better yet, use a digital thermometer to check it has reached setting temperature of 105°C / 220°F.