First, find a recipe , one that isn't this one <i>unless</i> prefer sweet, ultra mild BBQ sauce that without intersession would double nicely as a marinade or an aperitif. Otherwise, do more research on common recipes and see about adding watermelon to them.
Anyway. Second, get a watermelon, preferably a big one.
And then start chopping and seeding <i>6 pounds of watermelon</i> . I'm thinking now that it would be smarter to pulp the watermelon prior to putting on the heat, I believe that it would help it cook down more efficiently and make for a better sauce.
I have no idea if this is 6 pounds but the pot seemed to weigh less than a healthy full term newborn and about half of my cat. By this scientific method, I say 6 pounds. Little known fact, as raw watermelon cooks, it smells like cookie dough.
Its hard to get a weight of a watermelon because the weight includes the rind. The rind is heavy. I do not have a scale in the house and even if I had a kitchen scale, your average kitchen scale isn't set up for pounds of food.
Next. Cook that watermelon for <i>hours</i>. 2-3 Hours on low. At that point, add the rest of the ingredients ( tomato paste, onion powder, garlic powder, brown sugar, dry sherry, lemon juice and liquid smoke). Taste it, panic because it <i>still</i> tastes like sugar water and add about three tablespoons of cumin and three tablespoons of red pepper flakes. If that is too much spice, add some molasses, if <i>still</i> too mild, add more until you get what you want.
Uncover and cook on medium for an additional three hours. It is messy.
Add corn starch until mixture morphs from a marinade to a sauce. Remove from heat and cool,
Separate into smaller containers and freeze.
Its not a <i>bad</i> sauce. It has an unoffensive, ultra mild taste that might appeal to a mid western sensibility. I added dried red peppers to it and more garlic and onion powder. I should have added cumin and if I had pepper flakes I would have made liberal use of them. Its a nice sauce, its not going to burn your mouth or take away from the taste of the meat, I found it was very nice in stir fry and was surprisingly good with fish. Send me your snail mail addy and I'll send you some sauce.