Chop watermelon into large 1-inch cubes. Divide the watermelon in half and put one half in the blender. Puree.
Heat oil in a large skillet or wok.
Add cumin seed until they start to change color.
Add ginger garlic paste and stir fry for 30 seconds.
Add watermelon juice and start to cook on medium-high heat.
Add chili powder, turmeric, coriander. Stir to combine.
Bring to a boil. Reduce heat to a simmer and cook on medium low heat for 10 minutes.
Add chopped watermelon, sugar , salt and lemon juice.
Cook on low heat for 5 minutes, tossing well to coat the watermelon with the spices.
It is very important to only let the curry simmer for a few minutes after you add the diced watermelon to the curry. You don't want the chopped watermelon to take on a pulpy consistency, which will occur if it is overcooked.
For a more gourmet appearance, you can use a melon baller and make melon balls instead of just chopping the watermelon.