Cut the tomatillos into quarters.

Heat the oil in a large heavy sauté pan over medium high heat and in it sauté the onions for a few minutes and then add the tomatillos, garlic, and jalapenos.

Sauté until the onions and garlic begin to brown.

Reduce heat to low and sprinkle the cumin and chili powder over the pan.

Stir in the tomato sauce and bring to a simmer.

Remove from heat. Cool.

Place in a blender or food processor with the watermelon and puree.

Season with salt and pepper to taste.

Source:

Watermelon.org