When I was growing up, I was blessed to have my grandparents live in a downstairs apartment in our house. My grandmother would cook dinner (the noon meal) for whoever was there or happened to drop by. The leftovers were saved for supper (the evening meal) for she and my grandfather and for my brother and me as snacks when we got off the school bus in the afternoon. This simple dish was one we looked forward to along with her fried okra. Mama Tomerlin raised her 9 children and blessed her 36 grandchildren with her wonderful "scratch cooking" - a cook who could go into the kitchen and make a delicious meal from whatever was on hand.
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.