Combine water and brown sugar in a microwave-safe dish; microwave at HIGH for 3 1/2 minutes. Stir to dissolve sugar. Cool. Peel and pit peaches. Combine sugar syrup and peeled peaches in a food processor; process until smooth. Pour peach mixture into a pitcher; stir in white wine Grand Marnier. Chill at least 4 hours; strain mixture through a cheesecloth-lined sieve. Squeeze cloth to extract juices. Discard solids. Stir in cava. Garnish, if desired.