Prepare boiling water canner. Heat jars in simmering water until ready for use. Do not boil. Wash lids in warm soapy water and set bands aside.
Wash and dry the oranges and lemon. Slice the oranges into ⅛” slices and remove any seeds.

Stack the orange slices and cut into small pieces, about the size of a pea. Tip: I found cutting the slices into small pieces easiest to do with kitchen scissors!

Place the orange pieces and any reserved orange juice into a large pot. Add the lemon juice and zest.

Add the water and bring the mixture to a boil for 10 minutes. Reduce the heat to medium and simmer for 60 minutes, stirring occasionally. This softens the orange peels. A quicker method than soaking overnight.

After 60 minutes, add the sugar, stir to dissolve, and bring the marmalade to a full rolling boil. Once a full rolling boil is achieved, stir constantly to prevent burning until the mixture reaches 220 F. Use a candy thermometer to monitor this process, it will take about 20 minutes to reach this point. It will clearly thicken and become gel like.

Ladle hot jam into a hot jar leaving a ¼ inch headspace. Remove air bubbles. Wipe jar rim. Center lid on jar and apply band, adjust to fingertip tight. Place jar in boiling water canner. Repeat until all jars are filled.
Process jars 10 minutes, adjusting for altitude. Turn off heat, remove lid, let jars stand 5 minutes. Remove jars and cool 12-24 hours. Check lids for seal, they should not flex when center is pressed.