Wash the grapefruits thoroughly.

Use a vegetable peeler to remove the zest (outer skin), avoiding the white pith. Slice the zest into thin strips.

Peel off the white pith from the grapefruits and discard.

Segment the grapefruit, removing seeds and any remaining membranes. Collect the juices in a bowl.

In a large pot, combine the grapefruit segments, zest, and juice.

Add the juice and zest of the lemon.

Pour in the water and bring the mixture to a boil over medium heat.

Reduce the heat to low and let the mixture simmer for about 45 minutes, or until the fruit is soft and the liquid has reduced slightly.

Stir in the sugar, ensuring it dissolves completely.

Bring the mixture back to a boil and continue cooking until it reaches the gel stage (220°F/104°C on a candy thermometer).

Remove any foam that forms on the surface to ensure a clear marmalade.

While the marmalade is still hot, ladle it into sterilized jars, leaving about ¼ inch of headspace.

Wipe the rims, seal with lids, and process in a boiling water bath for 10 minutes.

Let the jars cool at room temperature.

Once sealed, store in a cool, dark place for up to 1 year. Open jars should be refrigerated and used within 3 weeks.