Once the onions are thoroughly softened, add the diced potatoes, season with salt, and stir well. Allow them to brown for a couple of minutes, then pour in the crushed tomatoes along with a splash of water used to rinse out the tomato bowl. The potatoes should be almost completely submerged.
Add freshly ground black pepper, cover with a lid, and cook for 40–45 minutes. Stir occasionally to ensure the potatoes stay moist and the sauce does not reduce excessively (if it begins to dry out, simply stir in a small splash of water).
Finish by stirring in a generous amount of fresh parsley, thyme, and marjoram leaves just before serving.