Toss the chopped strawberries, sugar and lime juice in the pitcher of a blender or in a deep bowl and leave at room temperature to macerate for about 15 minutes until the sugar dissolves, and the berries start releasing their juices.
Puree in the blender or with an immersion blender until smooth. Taste for sweetness—adding more sugar or lime, if desired. (I prefer my cocktails on the tart side.)
Once it tastes right, pass the puree through a fine-mesh sieve using a spatula to get rid of the strawberry seeds. Chill until ready to use.
Pour the strawberry puree into a tall glass. Add the tequila, lime and grapefruit juices and stir gently.
Transfer to your favorite glass, add ice and top with Topo Chico, to taste.