Put the yogurt in a wide cheesecloth-lined sieve set over a bowl. Cover, refrigerate, and let drain until you have 2 cups of very thick yogurt, about 12 hours. You can speed the process a little by occasionally using a rubber spatula to lift and turn the yogurt.
Put the milk and the saffron in a small saucepan or butter warmer and place over low heat until the milk is hot but not simmering. Cover and set aside to steep for 1 hour.
Put the drained yogurt in a bowl with the sugar, cardamom, cinnamon, and nutmeg. Whisk well to dissolve the sugar. Add the saffron milk 1/2 teaspoon at a time, whisking it in and tasting as you go. You will probably need only about 1 teaspoon; the saffron flavor should be subtle and it will bloom as the dessert chills. (Reserve any remaining saffron milk for cooking rice.) Cover the yogurt and chill thoroughly for up to 1 day.
To make the topping
Preheat the oven to 325°F.
Place the coconut, almonds, and pistachios in three separate pie tins.
Place in the oven. Bake the coconut until golden, about 4 minutes, stirring partway through so it toasts evenly; do not allow it to become dark brown or it will be bitter.
Bake the almonds until golden brown, stirring once so they cook evenly, about 5 minutes.
Bake the pistachios until fragrant, 5 to 7 minutes.
Remove each from the oven as it is done and let cool.
Coarsely chop the pistachios.
Combine the cooled ingredients in a small bowl.
To serve, divide the pudding among 4 serving glasses. Garnish with the topping. From Yogurt: Sweet & Savory Recipes for Breakfast, Lunch, and Dinner, by Janet Fletcher (Ten Speed Press, April 2015)
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