Nick Malgeiri, a well-known cooking instructor and pastry chef, shared this recipe with me after he made marmalade out of the oranges and lemons from my citrus trees. This can be made with any combination of citrus: all orange, all lemon, or 2 parts orange and 1 part lemon.
Makes about 1 quart
1 pound sweet oranges
½ pound lemons
6 cups water
3 pounds sugar
Rinse the fruit and trim away the stem and blossom ends. Halve and slice thinly, picking out the seeds and setting them aside. For very seedy oranges, halve, then cut away the core which eliminates most of the seeds.
Put the sliced fruit and any juice from the cutting board, and the seeds tied into a cheesecloth bag into a large non-reactive saucepan and add the water. Bring to a boil (careful, it boils over easily) and lower to a simmer. Cook 1 hour. Here the process can be interrupted until the next day.
Add the sugar and bring to a boil, stirring. Lower to an active simmer and cook to 220°F on a candy thermometer or instant-read thermometer, skimming often and stirring occasionally, more often as the marmalade thickens, about an hour. While the marmalade is cooking, pick out any stray seeds with a teaspoons; cooking darkens the seeds so they are easy to spot.
Pack the hot marmalade into sterilized jars. Cover with sterilized lids and cool. Store in the refrigerator for up to 6 months.