Cut the fruits in half. Use a soup spoon to scrape the pulp out. Remove any seeds and coarsely dice the grapefruit pulp and scrape it, along with any juice on the cutting board, into a bowl. Set aside.
Cut the remaining four grapefruit halves in half, remove the seeds, and use a sharp knife to cut the grapefruit in slices as thin coin size pieces.
Put the grapefruit slices in a large oven proof pot. Cover the grapefruit slices with water, add a pinch of salt, cover the pot, and bring to a boil.
Reduce the heat to a simmer and cook for 5 minutes. Turn off the heat and let the grapefruit slices sit in the hot water until they’re soft and translucent. Strain the grapefruit slices, discarding the water, and place the cooked grapefruit slices back in the pot along with the chopped grapefruit pulp, 5 cups of water, sugar, and lemon juice.
Cook the mixture over medium-high heat until it reduces and the marmalade has thickened and become syrupy. If using a candy thermometer, it’ll read around 102-103ºC.
Ladle the hot marmalade into clean jars and cover.
