Add chopped mangos, juice of ½ lemon and water to a pot, along with the rest of the squeezed lemon (skin and all).
Cover and cook over medium-low to low heat for about 30 minutes.
Once it starts to boil, reduce heat, uncover, and cook 60-90 minutes, until thickened. Mash the fruit with a wooden spoon and stir regularly, monitoring so the jam does not burn or stick to the bottom of the pot.
You can test the jam by spooning hot jam onto a frozen plate. If it’s done, the jam will instantly set.
Discard lemon skin and transfer mango jam to a glass jar or airtight container. Allow to cool before covering and storing in the fridge for 3-4 weeks.
(Or use a sterilized canning process for longer-term storage—up to 2-3 months.)