Preheat the oven to 450 degrees.
Peel and cube the potatoes (pieces should be about 1½ inch cubes). Place the potatoes into a large pot. Sprinkle on the salt and baking soda. Add cold water to cover the potatoes. Place the pot of potatoes on the stove and turn on the high heat. Cover the pot. Bring the water to a rapid boil and cook the potatoes for 10 minutes or until fork tender. (Remove the lid once the water is boiling and if necessary, reduce the heat to avoid the water boiling over.)
Drain the potatoes well and return the potatoes to the hot pot without the lid. (This helps the excess moisture to evaporate off the potatoes.) Allow the potatoes to steam off the excess moisture for a few minutes. Then put the lid on and shake up the potatoes hard. (Roughing up the parboiled potatoes will give them more crunch once they are roasted.)
Put the duck fat into a large deep cast iron pan and place it into the oven for about 5 minutes, or until it is just starting to smoke. When the duck fat is melted, add the potatoes to the skillet. (It is best for them to cook in a single layer, using 2 skillets if necessary.) Place the skillet back into the oven and allow the potatoes to cook undisturbed for 30 minutes. Then turn the potatoes over and allow them to again cook undisturbed for another 15 minutes. Turn them again, and allow them to cook for another 15 minutes or until golden and crunchy.
When you remove the potatoes from the oven, sprinkle them with seasoning salt, pepper, garlic powder, and onion salt. Serve them hot and fresh out of the oven.