2 lemons, quartered
1 cup fresh mint leaves
1/2 cup packed light-brown sugar
1/2 medium seeded watermelon (about 9 pounds), rind removed, flesh cut into chunks
1 cup vodka (optional)
Squeeze lemons into a large pitcher. Add squeezed lemon quarters. Add mint and sugar. Mash with a wooden spoon until mint is bruised and sugar is dissolved. In a blender, puree watermelon in batches until smooth. Pour through a fine-mesh sieve into pitcher (you should have about 8 cups of juice). Stir to combine. (Refrigerate covered up to 3 days.) Add vodka (if using). Serve over ice.