I saw this treat some time ago on Stumble Upon and wanted to try it. The original link is below and my recipe is different from their recipe but the technique is the same so be sure to visit their page.
My version is watermelon lemonade flavor.
Scoop out all the insides and process it in a food processor until smooth. Keep the rinds and place them in the freezer while the frozen lemonade is being made.
Place in pot with the water, lemon and sugar. Boil for 2-3 minutes until the sugar is melted and everything is incorporated. Place into a bowl and put into the refrigerator until it is completely cool.
After the watermelon lemonade is cooled, freeze it in an ice cream freezer per the instructions. To make it appear more realistic, I cut the seed pieces off of two blackberries and stirred them into the frozen lemonade. The original posting used mini chocolate chips. Or you can leave it without any “seeds”. Fill the mixture back into the frozen rinds.
Place them back into the freezer until they are frozen solid, a few hours. If you cut into them too early, the slush will run out and there isn’t a second chance to do it again. After they are frozen, use a big sharp knife to cut them into pieces.
Link to original:
Frozen Watermelon Lemonade Treat
1 Small Watermelon
1/2 Cup Sugar
1 Large Lemon
2 Blackberries (optional)
Cut the small watermelon in half and scoop out all the insides and process the pulp in a food processor. Save the rinds and place them in the freezer. In a pot, place the processed pulp, sugar and the juice from the lemon. Bring to a boil for 2-3 min and then remove and cool completely in a refrigerator. Place the contents into an ice cream maker and follow the directions for sorbet. After the watermelon lemonade is frozen, stir in the seeds from the blackberries (just cut the bumps off the core with a knife). Place the contents into the frozen rinds and return to the freezer until frozen solid (3-5 hours). Slice and serve frozen. They will melt quickly!