Combine all ingredients (except the ice and garnish) in a cocktail shaker and shake for 15 seconds. Then top up the shaker with ice and shake again for another 15 seconds or so.
Taste the drink for the right balance of sourness to sweetness, adding more agave or lemon juice if required.
Fine strain through a small sieve into a chilled glass (a Champagne coupe or rocks glass would work nicely).
Garnish with the rhubarb and a small sprig of rosemary.