In a large pot, put butter and celery, on low, so it just sweats and doesn't brown. Salt and cook for a few minutes and add onions. Salt onions and cook for a few minutes and add potatoes. Salt again and cover with water about 3" above potatoes. Raise heat to medium and boil til potatoes are fork tender, probably about 15 minutes. Add milk. Mix cornstarch and water into a slurry. When you see milky water is just coming back to a boil, add your slurry, stirring constantly. It should thicken within 1 minute, when it comes to a bubble it should be done. If it's too thin, mix up a little more but it probably won't be. If it's too thick, add a little whole milk. The buttery oil that was floating on top should be totally incorporated and no longer visible. That's when you know it's the right thickness and done. Taste, it will probably need a little more salt and this is when you add pepper. Potato soup takes a lot of salt.