- Defrost raspberries. Defrost either overnight in refrigerator or on low setting in microwave. Keep temperature of berries under 70°F.
- Bring to boil water, sugar, and agave syrup. Boil for 2 minutes and remove from heat. Pour in bowl and place in freezer until very cold.
- Puree raspberries until very smooth puree is obtained.
- Press puree through sieve and discard seeds. Obtain a total of 3 cups raspberry puree. Use any remaining puree to flavor drinking water.
- Combine puree with cold syrup and cold heavy cream and whisk smooth. Place in freezer until very cold.
- Meanwhile prepare ice cream machine (follow manufacturer's directions). Pouring very cold ice cream mixture into machine will shorten churning time and provide smoother and better ice cream. (Gelato, a lower fat version of ice cream, churns in professional machines in just 2-3 minutes, but feels very rich on the tongue.)
- Spread churned ice cream into a 9-inch pan. Return to freezer.
- Allow to firm for at least 4 hours or overnight.
- Scoop and serve with fresh mint sprigs, if desired.