Fill a medium sized saucepan 3/4 of the way with water. Add about 2 tsp salt. Bring to a boil and add frozen peas. Keep the stove temperature on high and once water is again boiling, add the potatoes. Reduce heat to medium. Boil potatoes & peas for about 15 minutes- until the potatoes are fork tender, but not falling apart. (Check often after you've boiled them for about 12 minutes - you don't want them to get too soft!)
Drain potatoes & peas and set aside.
In the same saucepan, melt butter. Add onion powder and flour. Quickly begin whisking in milk as the flour-butter mixture thickens, adding the milk a little bit at a time. Allow the milk to heat up and the mixture to thicken again before adding more, until all the milk is incorporated. Keep whisking until the mixture is smooth and creamy. Add a bit more milk if it gets too thick. Add in Parmesan cheese and whisk gently, until cheese is mixed in and melted.
Add the potatoes and peas back to the saucepan. Stir gently to combine. Serve with additional Parmesan cheese sprinkled on top.