Dice one large onion (and celery, if using) and put in bottom of large stock pot with approximately 1 Tbsp of oil and/or butter. Cook over medium heat, stirring regularly until cooked (approximately 10 minutes).
Wash, peel and cut potatoes and carrots into large pieces.
Add carrots and potato to the pot and cover with water (approximately 4-6 cups).
Add in stock cubes according to directions on box (we use 1 cube per 2 cups of water).
Bring mixture to a boil and switch to a low simmer.
Leave on a low simmer for at least an hour, but you can simmer it all day if you wish.
Remove soup from heat source and let stand for a few minutes.
Mix/blend soup together until smooth using an immersion hand blender or a stand blender.
Serve soup with a small handful of shredded cheddar cheese in each bowl that can be stirred to combine into the hot soup.
You can serve this soup with a small swirl or dollop of sour cream on top.
This soup goes really well with Homemade No Knead Bread.