Rinse (uncooked) chickpeas in a fine mesh strainer and add to a large pot. Cover with 2 inches water and bring to a boil over high heat. Boil for 1 minute. Then cover, remove from heat, and let set for 1 hour. Then drain and lightly rinse and dry thoroughly (method from the Kitchn).
In the meantime, add parsley, onion, and garlic to a food processor. Mix until well processed. Set aside.
Once the chickpeas are slightly cooled and dried, add to the food processor, along with gluten-free oat flour, salt, cumin, cardamom, coriander, and cayenne (optional). Mix to combine thoroughly, scraping down sides as needed. This may take up to 4-5 minutes to fully incorporate all of the spices and herbs. You're looking for a near paste with only very small bits of chickpeas and herbs.
Sample a small bit and adjust spices/salt/herbs as needed. Then cover and set in the refrigerator for 1 hour to allow the flavors to meld and the texture to become more firm.
Once cooled, remove from refrigerator and scoop out 1 1/2 Tbsp amounts (using this scoop or a Tablespoon) and gently form into small discs using your hands. If the falafel isn't sticking together the mixture may need to be blended more thoroughly, or dusted with a bit more oat flour if too wet.
Once your falafels are formed (~24 total // amount as original recipe is written // adjust if altering batch size), heat a large metal or cast-iron skillet over medium/medium-high heat. Once hot, add enough oil to create a thin layer on the bottom of the pan and wait 1-2 minutes for the oil to heat up. Then place only as many falafel as will fit comfortably in the pan and cook 2-3 minutes on each side. Once the bottom side is browned, flip gently as they can be fragile. Continue until all falafel are cooked. Adjust heat as needed if they're browning too quickly, or aren't cooking quickly enough.