Start by cutting a ripe seedless watermelon into small, bite-sized cubes. Spread these pieces evenly on a lined baking sheet and place them in the freezer for about 3 hours.

In a medium saucepan, combine 1 cup of sugar with 2 cups of cold water. Heat the mixture over medium heat until it reaches a gentle boil. Simmer for about 3-5 minutes until the sugar has completely dissolved. Let it cool.

In a large pitcher, combine 3 cups of cold water with freshly squeezed lemon juice to taste (about 1 cup). Add the cooled sugar syrup to this mixture, stirring well. Chill until your watermelon cubes are frozen.

When the watermelon is fully frozen, take out 2 cups of watermelon cubes and add to a blender. Pour in 1 cup of the chilled lemonade mixture and blend on high speed until smooth and slushy.

Pour the Frozen Watermelon Lemonade into tall glasses and garnish with lemon slices and fresh mint leaves. Serve immediately.