Slice the watermelon. Chop the red part to make up to at least 1 cup or more.
Peel off the green part. Finely chop the white part till you have 4 cups full.
Peel, core and chop the green apple into pieces.
Add the white and red watermelon along with the apple into a wide pan that you will use for cooking the jam.
Add lemon juice, lemon rind and sugar to the mixture. Mix well.
Allow the mixture to rest for 30 minutes. This will allow the sugar to melt and the liquid will help in cooking the watermelon. If you want to, you can put the mixture in the fridge and leave it overnight.
Heat the mixture over medium to high heat till it comes to a gentle boil.
Lower the heat and allow the mixture to simmer. You can cover the pot so that the watermelon rind cooks faster.
In between give the mixture a gentle stir so that it does not stick to the bottom of the pan.
When the rind appears translucent and soft, add the grated beetroot. This takes about 20-30 minutes over medium heat.
Blend the mixture with a hand blender. You can blend it to a smooth consistency or a slightly coarse one. I left mine a bit coarse.
Cook till the water disappears completely and the mixture is thick. Mine took about 20 minutes.
Add cardamom powder and mix well.
Spoon the jam into a sterilized jar or bottle. I usually sterilize the bottle in the microwave oven for 2-3 minutes.