Cut peeled potatoes into 1/2-3/4 chunks and place in a large dutch oven or saucepan. Cover with water.
Cook until fork tender, about 15-20 minutes. Drain and leave in strainer. Remove about 1/2 of the potatoes and mash. This will add a bit more thickness to the soup.
In the same pan, melt butter and add chopped onions and celery. Saute for about 8-10 minutes until celery is soft and onions are translucent.
Sprinkle with flour, stir and cook for a minute or so.
Cook over low/medium heat until hot then add velveeta and cheddar cheese. Stir.
Once cheese has melted, add in the potatoes that have been mashed, stir and then add the remaining potato chunks to the liquid.
Continue on a low heat until ready to serve.
Top with additional cheese if desired.
Recipe adapted from The Magical Slow Cooker.