In a bowl, whisk together the boiling water, brown sugar, ginger, and tamarind paste until the mixture is combined well. Let steep for 5 to 6 minutes, then add the lemon juice and let the mixture cool. Strain it through a fine sieve set over a pitcher and chill for 2 to 3 hours. Divide the cooler among tall glasses filled with ice cubes and a mint sprig.
Recipe courtesy Bobby Flay