1. Pour boiling water over tea bags, sugar, and lemon zest in a large bowl. Stir until sugar is dissolved; cover and steep 5 minutes.
2. Pour mixture through a fine wire-mesh strainer into a large pitcher, discarding tea bags and zest. Stir in 4 cups cold water and next 2 ingredients. Cover and chill 30 minutes to 12 hours. Serve over ice. Garnish, if desired.
To make this tea a day ahead, follow recipe as directed, omitting ice and garnish. Store in the refrigerator in a pitcher. When you are ready to serve, stir in ice cubes and lemon slices.