If desired, preheat grill. Place lemon halves, cut sides down, on the rack of an uncovered grill directly over medium heat. Grill for 5 minutes or until cut sides are charred. Remove and cool.

Meanwhile, in a medium saucepan combine 1 cup water, sugar, lavender, and rosemary. Bring to boiling, stirring to dissolve sugar. Remove from heat. Let stand until cool. Pour through a sieve; discard solids.

Juice the lemon halves. In a pitcher combine the lemon juice, syrup, and remaining water. Chill until needed. Serve over ice topped off with sparkling water and with additional rosemary and lavender, if desired.