Using a mortar and pestle or a heavy pan, crush the nutmeg into pieces. With a spice grinder, grind the crushed nutmeg into a powder and set aside.
In a frying pan over medium-low heat, add all the other spices (EXCEPT the nutmeg and cardamom if your cardamom is not shelled) and toast for 2 to 3 minutes, or until you see the fennel turn a slightly darker shade and the spices become fragrant.
Remove the pan from the burner and allow the spices to cool.
If using whole cardamom pods, toast the same way you toasted the rest of the spices. Allow to cool. Using a spice grinder, give it a short whirl or two and most of the shells will open and be separated from the seeds. Or, if using a mortar and pestle, coarsely crush the cardamom pods. Pick out the seeds and discard the shells. Grind the seeds to a powder and add to the rest of the spices.
Store the Kerala garam masala in an airtight container and it will keep for 1 to 3 months. If you store it in the refrigerator or the freezer the spice blend will be good to use much longer. Smelling the masala gives you a good idea of whether it is still fresh enough to flavor your cooking.
Be sure to buy good quality spices for Kerala garam masala.
When toasting the spices, use medium-low heat so as allow the essential oils from the spices to be activated before they get too brown.