Cut the oranges in half. With a spoon, remove the flesh. Set aside the peels. In a blender, process the orange flesh with 1 cup (210 g) of the sugar. Pour into the same saucepan. Add 1 cup (250 ml) of the reserved cooking water.
Bring to a boil and let simmer for 10 minutes. Strain, pressing the mixture to extract a maximum of liquid. Set aside the strained orange syrup and discard the pulp.
Meanwhile, thinly julienne the peels, and cut them into 3 pieces. Place in the saucepan.
Add the orange syrup, the remaining sugar (2 cups / 420 g), the lemon zest and 2 cups (500 ml) of the reserved cooking water.
Bring to a boil and let simmer for 1 hour or until a candy thermometer reads 223°F (106°C). Skim several times while cooking.
Transfer into airtight containers or hot jars. To sterilize jars of marmalade, completely cover with water and boil for 10 minutes. Sterilized jars will keep for 1 year at room temperature. Unsterilized jars will keep for 1 month in the refrigerator.
Note from Ricardo
During our tests, we loved working with navel oranges—medium-sized with sweet and oftentimes seedless flesh.