Clean your sweet potatoes well, remove the ends if necessary and cut them into long, thin and uniform strips, about 1/4" thick.
Place the potato strips in a bowl and fill it with cold water. Let the potatoes soak in the cold water for at least a half hour, but no longer than 24 hours. If soaking for more than a few hours, be sure to place the bowl in the fridge.
When ready to bake your fries, place a large baking sheet in the oven and turn the oven on to 475°F.
Drain the potato strips, rinse them well and then pat them really dry with a clean tea towel. Put the potatoes into a clean, dry bowl and drizzle them with the avocado oil; Toss until all the strips are evenly coated.
In a separate bowl, combine the corn starch, nutritional yeast, garlic powder, paprika, ground pepper and red pepper flakes (if using) and mix with a small whisk until thoroughly combined.
Sprinkle over the oil-coated potato strips and then toss until all the strips are evenly coated with the spice mixture.
Carefully take the pan out of the oven and decrease the temperature to 450°F.
Carefully line the pan with parchment paper and on it, carefully arrange the potato strips in a single layer, making sure that they don't touch or overlap. Did I say carefully? Be careful. That pan is hot!
Bake the potatoes for 15 minutes, then remove the pan from the oven and flip each fry individually with a pair of kitchen tongs. Again make sure that the fries are in a single layer and do not touch or overlap.
Resume baking for 10-15 more minutes, or until the fries start to crisp up and take a nice golden coloration. Feel free to flip the fries once again and to return them to the oven for a few more minutes if you feel they could use a bit more baking time.
When cooked to your liking, remove the fries from the oven and sprinkle with salt, then let them cool in the pan for about 5 minutes before serving. Garnish with chopped green onions or parsley, if desired.