Using a sharp knife, slice the tops and bottoms off all the fruit. Then, slice away the peel and pith, getting as much of it as you can without cutting into the fruit.

You won't need the peel for the recipe, but don't throw it away - the zest can be used in so many ways!

Slice the blood oranges thickly and remove any seeds.

For the pummelos, you'll use that sharp knife to cut in between the membranes to release the flesh - remember to do it over a bowl to catch any fruit. Discard the membranes.

Add the blood orange and pummelo to a food processor and puree in batches.

Add the pureed fruit to a large pot, and stir in the sugar and lemon juice. Drop in the rosemary.

Bring to a boil, and lower the heat as you continue to simmer for 30 to 40 minutes. The mixture should thicken and reduce by 2/3 or so. Stir more frequently as it thickens, as it can scorch on the bottom of the pot.

You can leave the texture as is, or push it through a sieve if you prefer a smooth jam.

Let the jam cool for 5 to 10 minutes, remove the rosemary sprigs, and then transfer to clean jars.

Allow the jam to cool completely at room temperature before covering.

Tightly covered, it will keep up to three weeks in the fridge, or up to a year in the freezer.