Cut a wedge of lime and wipe it around the edge of your Margarita glasses. Dip the glasses into a plate filled with ground Himalayan pink salt and set aside.
Put the raspberries, agave nectar and lime juice in a cocktail shaker and muddle them with a muddling spoon or the end of a rolling pin. Strain the raspberry pulp through a small sieve and press down with the back of a spoon to extract as much juice as possible from the berries. If you want a really smooth cocktail, strain one more time.
Put the tequila and raspberry syrup in a cocktail shaker half filled with ice and shake hard for 30 seconds.
Strain into the prepared glasses, top up with chilled Champagne or Prosecco and serve.