Spiked Watermelon Cooler/Slushie

Serves 4 to 6

6 cups cubed seeded watermelon, 3/4-inch cubes
1 Tbsp freshly squeezed lemon juice
1 tsp freshly grated lemon zest
1 cup tonic water
1/2 cup vodka (optional)
6 Tbsp powdered sugar, or more to taste

If making into a slushie, place chunks side by side on a large baking sheet, so that they're not touching one another. Freeze at least 2 hours. Then puree melon cubes (frozen or not) with the remaining ingredients in a conventional blender or using a stick immersion blender. Taste and add more powdered sugar, if desired. For slushie, pour into glasses or small bottles and serve immediately. For non-frozen drink, pour into a pitcher and chill well before serving.