Place the millet and the water in the rice cooker or Instant Pot and set to the "rice" setting.
While the millet cooks place 2 tbsp of the coconut oil in a medium saute pan. Add the cassava, mushrooms, garlic, scallions, radishes, zucchini, and salt. Saute over medium heat for five minutes. Turn off the heat and allow to cool for five minutes. The cassava may stick to the pan, but if you leave it to sit it makes it easier to release from the pan.
Once the millet is done, carefully release the pressure and remove the lid. Add the sauteed vegetables to the millet, and stir to combine. Preheat the oven to 375 °F.
Put the remaining coconut oil on a sheet pan, and spread it out if it is solid. Using a ⅓ measuring cup, scoop the vegetable mixture into mounds on the pan, flatting them to about 1-inch thick using the back of the measuring cup. Continue making the patties thus, spacing them about two inches apart.
Place the pan in the oven, and bake the patties for 30 minutes. Remove them from the pan and serve immediately with tartar sauce, or freeze individually for later consumption.