This is a modification of the Raspberry Fizz by Sebastian Reaburn found in Food & Wine Cocktails 2010. I didn’t have raspberries to hand, but blackberries were. I’ve made one, for the wife. She would let me make another.
Muddle eight blackberries in a shaker. Add ice, 1 oz. of lemon juice, and a squirt of agave nectar. Shake well. Strain into a highball glass and fill with seltzer water. (I just tossed the mix in, since the liquid wasn’t straining past the blackberries.)