Combine the raspberries and lemon juice in a food processor

Process until very smooth

Pass through a chinois, pressing lightly on the fruit with the back of a spoon

You want as much juice as possible to pass through the sieve, but none of the solids

Discard the solids and place the juice into the fridge overnight

Mix the juice with the simple syrup

Churn in your ice cream maker until thickened

Freeze for at least 2 hours before using