Upside-Down Martinis for a Crowd

Combine 4½ cups dry vermouth and 1½ cups gin in a large freezer-safe airtight container (two 1-qt. containers work well). Freeze at least 3 hours and up to 1 week.

One hour before serving, remove container from freezer and stir in olive brine (if using) to taste and 1 cup water. Return to freezer and chill the remaining hour. (Mixture won’t freeze in that time because of the alcohol content, but it will if left in the freezer much longer.)

To serve, divide cocktail among glasses and garnish each with green olives or a lemon twist if desired.