Slice one side of the mango about 1/4 inch away from the stem. Do the same on the other side of the mango. Score a grid-like pattern on the flesh without cutting all the way through the skin. Use a spoon to scoop out the flesh in a bowl. Cut vertically on both sides of the seed, using a paring knife, gently remove the flesh and add it to the bowl. Discard the seed.
With the back of a fork mash the mango pulp, leaving little chunks of mango for added texture or transfer the mango pieces into a blender and blend to a smooth puree, without adding any water.
In a heavy-bottomed pot, add mango pulp, sugar, and lemon juice, heat it over medium-high heat and mix well with a wooden spatula.
Once the mixture starts bubbling reduce the heat to medium, and stir continuously until the mixture begins to leave the edge of the pan. Keep checking as you stir, drop the mixture from the spatula, it will drop easily, which means it needs to be cooked further. (about 45-50 minutes)
Once all the moisture has evaporated, the jam will begin to leave the sides of the pan.
If the jam does not drop from the spatula, it is done. Remove it from the heat.
If you want a slightly thinner jam then follow this test for jam doneness- Place 2-3 small plates in the freezer before you begin preparing the jam. When the mixture begins to thicken start testing. Remember to keep the pot away from the heat while you are testing. Put a tablespoon of jam onto a cold plate, return the plate back to the freezer for 2 minutes. Run your finger through the jam, if it wrinkles and stays apart (as shown in the pic) it is done, if it is runny, then it needs to be cooked for a few more minutes. Or use a candy thermometer, when the mixture reaches 220 F/ 105 degrees C, it is done.
Transfer the prepared jam to a sterilized airtight container and store in a cool and dry place.