Cut peeled potatoes into quarters. Place in saucepan and cover with COLD water. Add salt and bring to a slow boil on stove eye. Cook on low heat (cooking on high heat with cause them to mush and not keep their shape). Add onion slice for flavor, remove it later if you don't want to serve the onion in the potatoes. When tender, add butter and thickening. For thickening - Put flour in a small container, add small amt. of salt, add small amt. of cold water and stir into a thin paste. (You may need to add a bit more water.) Stir until all flour lumps are dissolved. Add to potatoes and bring to a low boil. NOTE: As with any thickening, whether you are using flour or cornstarch, you must bring it to a boil for it to thicken properly. Boil until liquid is at desired thickness. Add black pepper as desired.