Cut the potatoes into bite-sized pieces, if needed. Put them in a pan of cold water then bring to the oil. Cover and simmer for 10-15 minutes, until tender. Drain the potatoes and set aside.
Cut the onion in half, then into thin slices.
Heat the oil in a large frying pan over a medium heat. When hot, add the onion, turn down the heat and gently sweat the onion for about 10 minutes over low heat.
When softened, add the balsamic vinegar, marjoram and sugar, if needed. Stir for a minute or so until the onion has lightly caramelised, then add the potatoes and cook over low heat for 5 minutes or so, until the potatoes are nicely coated.
Crumble the cheese on top, put a lid on, turn off the heat. Rest for 3-5 minutes for the cheese to melt.
Serve warm.