This recipe was developed by pastry chef Kim Boyce. Try it as a filling for her Marmalade Barley Scones.
Put a small plate into the freezer for testing the jam later. Scrub and dry the citrus. With a vegetable peeler, strip the zest (the outermost layer of the rind) from 1 orange, 2 blood oranges, and 2 Meyer lemons. Slice the strips into thin matchsticks about 1/16th inch in width. Add the strips to a small pot and cover with cold water. Over high heat, bring the pot to a boil and boil for 30 seconds. Drain the strips, rinse with cold water, and set aside.
Cut both ends of each fruit. Using the side of your knife, remove the white pith and peel of each fruit. Discard the peels. Cut the fruit into quarters, removing any seeds or hard, pithy centers. Chop each fruit into roughly 1-to-2-inch pieces, saving all of the juice that drips onto the cutting board.
Measure the fruit and any remaining juice (you should have about 6 cups.) Transfer to a medium heavy-bottomed pot. Add 6 cups water to the pot with the fruit and bring the mixture to a boil over high heat.
When the mixture comes to a boil, lower the heat to medium. Cook fruit syrup, uncovered, for 1 hour, or until it has reduced by half. Periodically skim off white foam that accumulates at the top, and stir occasionally.
Add the blanched zest and the sugar to the pot. Stir to combine, increase the heat to high, and bring the mixture to a boil. Reduce the heat to medium, and cook, about 10 minutes. The mixture should be at a bubbling simmer. After 10 minutes begin to stir the marmalade constantly. Cook for about 20 minutes more.
The marmalade is ready when it is thick enough that a spoon leaves a trail at the bottom of the pan.
Remove the plate from the freezer. Test the marmalade by placing a spoonful of it on the plate. It should thicken promptly. Prepare an ice water bath.
Carefully pour marmalade into a medium bowl, scraping any bits with a spatula. Set the bowl into an ice water bath. Let cool slightly, stirring, about 5 minutes. Cover surface with plastic wrap and let cool completely. Store in airtight containers in the refrigerator for up to 2 weeks.