Wash and peel the mangoes.
Cut the flesh into small cubes. Discard the seeds.
Place the cut mangoes in a deep saucepan.
Add sugar and strained lemon juice and cook over low heat.
Stir the jam every few minutes. The jam will be watery as the sugar melts.
Continue to cook over low heat until the jam thickens, about 20 to 25 minutes.
If you have a candy thermometer, cook the jam until it reaches 105°Celsius or 220°Fahrenheit and remove from heat.
If you don't have a candy thermometer, as soon as you see the jam thickening slightly, spoon some into a plate. Let it cool down and push it with your finger. If the surface wrinkles, the jam is ready. If not, let the jam cook for a few more minutes and continue the test.
The jam will thicken more as it cools down, hence, it is best to not let it thicken too much on heat.
Let the jam cool completely before spooning into a clean and lightly heated bottle.
Place the lid and keep the jam refrigerated.