Gently heat 150g of the sugar with 150ml water in a heavy-bottomed pan to make a sugar syrup. When the sugar has dissolved, boil for three minutes, remove from the heat and set aside.
Puree the raspberries and remaining sugar in a food processor.
Pass the puree through a sieve to remove the pips.
Add the sugar syrup to the puree, together with the lime juice and only one tablespoon of the tequila (too much alcohol will mean the nieve will never freeze properly).
Pour the mixture into a shallow freezer-proof container or tray and place in the freezer for about two hours until partially frozen.
After two hours, remove the nieve from the freezer and mix it up with a fork to break up any ice that is forming. Return to the freezer and leave for another hour. Repeat the process until you have a crunchy frozen nieve.
Serve the nieve in small bowls with a little extra tequila poured on top.