I see the instructions say add potato starch, but it's not on the ingredient list. 1) how much should we add? 2) Can we sub for cornstarch?

Hi! As noted at the end of Step 3, potato starch will naturally accumulate in the bottom of the bowl after squeezing out all the water from the potato/onion mixture. Let the liquid sit in the bowl for a bit, then pour out, but leave any thick milky white substance in the bottom—that's the potato starch!