Cut up the oranges to about 1 cm pieces, and put into a pressure cooker. If cutting is too much work, just divide into segments with your hands.
Put the granulated sugar and lemon juice in the pressure cooker, and mix about 30 times until everything is well combined. Use granulated sugar, or the final jam will have a bad color.
Lock the lid and bring up to pressure over medium heat. When it reaches pressure, turn the heat down to low. If you brought up the heat from low to high, cook for 2 minutes. If on low pressure, cook for 3 minutes. I cooked for 3 minutes.
Cook under pressure for 3 minutes, then turn off the heat. Leave it to cool down and de-pressure naturally.
When the lid can be unlocked, take it off and simmer over medium-high heat while mashing the orange pieces with the back of a ladle. It's ok that there's a lot of moisture in the cooker when you open it.
Done. Adjust the simmer time depending on how much you make. The longer you simmer it, the more moisture will evaporate and the thicker the jam will become. It will thicken even more after a few days.
Story Behind this Recipe
We received clementines as a gift, but they were rather sour. I turned some of them into jam before they went bad.
The final simmering over medium heat is to evaporate the moisture. Stop cooking when it is nicely thickened. Be careful not to let it burn. This recipe is good for rather sour oranges. For sweet oranges, use less than 100 g of sugar. If you don't use granulated sugar, it may end up too sweet.