Preheat oven to 375 degrees F.
Place sweet potatoes in a steamer basket over a pot of boiling water. Cover and steam until soft, about 12 minutes. Remove lid and let cool slightly.
Meanwhile, place the chopped onion, black beans, salsa, chili powder, cumin, garlic powder, salt and grated cheese in a large bowl. Stir to combine.
Add the cooked sweet potatoes to the bowl with the other ingredients and gently mash with a fork. Then, stir them into the black bean mixture.
Spoon about 1/3 cup of the mixture across the lower third of each tortilla and roll up tightly.
At this point, the taquitos can either be baked immediately, refrigerated for up to 1 day and then baked, or frozen. See note below for freezer directions.
To bake taquitos: place seam-side down on a parchment-lined baking sheet. Lightly brush each taquito with olive oil (or lightly spray with cooking spray), and sprinkle with kosher salt.
Bake for about 15 minutes, until golden brown.