Add dried Kashmiri chilies to a heavy-bottomed pan and dry-roast them (stirring occasionally) until fragrant. This will take about 6-7 minutes over medium heat.
Take it off the heat and set them aside to cool. Remove the stems and transfer them to a blender.
Blend until the chilies turn into a fine, smooth powder. Set them aside.
Combine the remaining spices (listed under whole spices) in a skillet and dry-roast over medium-low heat, stirring frequently until they smell toasty fragrant and slightly dark in color.
Take it off the heat and set them aside to cool. Transfer them to a blender.
Blend until the whole spices turn into a fine, smooth powder.
Combine garlic powder, ginger powder, nutmeg powder, salt, and beetroot powder in a large mixing bowl. Add the powdered whole spice mix to it.
Add powdered chilies to the bowl.
Mix until well blended.
Now, transfer this tandoori masala mix through a fine strainer.
Sift the tandoori blend through the fine sieve and discard what's left in the strainer.
Your homemade tandoori masala is ready for future use.
Store in an airtight container and good for 5-6 months.