Spread the cinnamon, cardamom, cloves, cumin, coriander and peppercorns in one layer in a large shallow roasting pan.
Roast on the bottom shelf of the oven for 30 minutes, stirring and turning the mixture 2 or 3 times with a large wooden spoon.
Do not let the spices brown.
Break open the cardamom pods between your fingers or place them one at time on a flat surface and press down on the pod with the ball of your thumb to snap it open.
Pull the pod away from the seeds inside and discard it. Set the seeds aside. Place the roasted cinamon sticks between the 2 layers of a folded linen towel and pound them with a rolling pin or a kitchen mallet until they are finely crushed.
Combine the cardamom seeds, crushed cinnamon, cloves, cumin seeds, coriander seeds and peppercorns in a small pan or bowl and stir them together until they are well mixed.
Grind the spices a cup or so at a time by pouring them into the jar of an electric blender and blending at high speed for 2 or 3 minutes, until they are completely pulverized and become a smooth powder.
If the machine clogs and stops, turn it off, stir the spices once or twice, then continue blending.
As each cupful of the spices is grounded, transfer it to a jar or bottle with a tightly fitting lid.
Garam Masala may be stored at room temperature in an airtight container, and will retain its full flavor for 5 to 6 months.