Drain the yogurt in a cheesecloth-lined sieve for at least 4 hours, or until it loses half its volume. Reserve the liquid for cooking or drinking.

Crack the cardamom seeds with a mortar and pestle or a rolling pin, then put them into a large mixing bowl. Add the yogurt and beat until smooth, then add the pistachios, nutmeg and sugar, and continue to beat for at least 10 minutes, until the sugar has dissolved. Taste and adjust the flavor, adding more sugar if needed to make the shir-kand sweet, but not overwhelmingly so. Beat in the rosewater, then cover and refrigerate for at least an hour. When it is quite cold, transfer the shir-kand to a serving dish and garnish with a few whole pistachios.