Skin your baked potatoes and chunk the insides; set aside.
In a large sauce pot, melt butter & saute your onions until translucent. Add your flour and stir to make a roux. Gradually add chicken stock, water and cornstarch.Stir add mashed potato flakes & spices. Bring everything to a boil and reduce heat; simmer 5 mins.
Add your chunked potatoes and half n' half to the pot.. bring to a boil once more; reduce heat and simmer 5-10 mins.
If your soup is too thick just add stock to thin it out.
Special note: I like to add crumbled bacon and shredded cheddar cheese to mine. Gives it a really good flavor. You could also use small pieces of ham, too.