Clean and wash the potatoes and cut into slightly larger quarters.
Heat the oil in a saucepan and brown the onion in it, then add the peeled and diced tomatoes, the garlic, salt, pepper, the potatoes and 1 1/2 cups of water and simmer until the potatoes are soft .
Stir every now and then.
The sauce should bind a little, if the potatoes are not yet soft, just add a little more water.