Wash the oranges and lemon thoroughly under running water, scrubbing the skins to remove any wax or dirt. Pat them dry. Using a sharp knife, slice the ends off the oranges and lemon and discard them. Cut the fruit in half, and then slice each half into very thin, half-moon shapes. From there, you can chop these slices into smaller, bite-sized pieces. The goal is to have small, uniform pieces of peel and fruit. As you chop, remove and discard any seeds you might find (Navel oranges should have few, if any).
Place all the chopped oranges and lemon into a medium, non-reactive saucepan (such as stainless steel or enamel). Pour in the water. Bring the mixture to a simmer over medium heat. As soon as it begins to simmer, turn off the heat, place a lid on the saucepan, and let it sit at room temperature for 24 hours. This soaking step is crucial—it softens the peel and extracts the maximum amount of natural pectin, which is the key to a perfectly set marmalade.
The next day, return the saucepan to the stove over medium heat. Bring the mixture back to a gentle simmer and cook, uncovered, for about 45 minutes to 1 hour. Stir occasionally. You are looking for two things to happen: the liquid should reduce by about half, and the citrus peels should become very soft and translucent.
Once the peels are tender, stir in the sugar until it completely dissolves. Increase the heat to medium-high to bring the mixture to a rapid, rolling simmer. Let it cook for 15-25 minutes, stirring frequently to prevent the sugar from catching on the bottom and burning. The marmalade will begin to thicken and look glossy.
This is the most important step. Before you start cooking, place a few small plates in your freezer to get very cold. To check if the marmalade is ready, turn off the heat. Spoon a small amount of the marmalade onto one of the frozen plates and place it back in the freezer for 1-2 minutes. After a couple of minutes, remove the plate and gently push the marmalade with your finger. If it wrinkles and feels gel-like, it’s done. If it’s still runny, turn the heat back on and continue simmering for another 3-5 minutes before testing again.
Once the marmalade has reached its setting point, remove it from the heat. Let it cool in the saucepan for about 10-15 minutes; this allows the peel to distribute evenly throughout the mixture instead of floating to the top. Carefully pour the warm marmalade into clean, airtight jars. Allow it to cool completely on the counter before sealing and storing it in the refrigerator. It will last for up to 6 weeks.