Dry-roast the coriander seeds in a small skillet over medium heat, sliding the skillet back and forth over the burner to prevent burning, until the seeds exude a pleasant aroma, 1 to 2 minutes.
Transfer to a bowl and set aside. Repeat the process, one at a time, with the cumin seeds, peppercorns, cardamom, cinnamon, and cloves.
Let cool completely.
I recommend you make up only enough to use within 1 month so it will remain fresh.
Put all the ingredients except the nutmeg in a spice grinder and grind to a fine powder. Transfer to a bowl. Grate the nutmeg over the mixture. Stir to mix well.
Transfer to a glass jar with a tight-fitting lid. The garam masala will keep at room temperature for at least a month.