Cut each orange in half. Lay each half flat side down and slice as thinly as possible, about ⅛ inch. Quarter the slices if you prefer shorter shreds. Remove the seeds as you go.

Add the sliced oranges, sugar, water, and lemon juice to the inner pot. Stir well so the sugar is fully moistened and none is sitting dry on the bottom.

Lock the lid and set the valve to Sealing. Cook on High Pressure for 10 minutes.

When the cook time ends, allow a full natural pressure release, about 10–15 minutes. Do not quick release.

Remove the lid and select Sauté. Bring to a steady boil and cook, stirring often, until the mixture reaches 220°F to 222°F on a candy thermometer, about 15–20 minutes. Stir more frequently as it thickens to prevent scorching.

Test the set: drop a spoonful onto a plate from the freezer, chill 1 minute, then push it with a finger. If it wrinkles and holds a line, it's ready. If not, cook 5 more minutes and retest.

Press Cancel. Let the marmalade rest in the pot 10 minutes so the peel stays suspended, then ladle into clean jars.

Cool completely on the counter, about 1–2 hours, before capping and refrigerating. The marmalade will continue to thicken for up to 24 hours.