Set the oven temperature to 400 degrees and bake the potatoes for an hour or until they are done.
Once the potatoes are cooked through, remove them from the oven and set them aside to cool.
While the potatoes cool, make the soup by melting the butter in a large saucepan and cooking the onion until it turns light brown.
Mix the onions and flour together to make a roux.
Bring to a boil the stock, water, cornstarch, mashed potatoes, salt, and spices.
Reduce the heat to low and keep it at a simmer for the next six minutes.
Cut the baked potatoes in half lengthwise and use a large spoon to scoop out the flesh. Discard skin.
Use a big knife to cut the baked potato into small pieces that are about 1/2 inch long.
Bring the soup back to a boil, then add the chopped potatoes and half-and-half.
Reduce the heat, and continue simmering the soup for an additional 15 minutes, or until it has reached the desired consistency.