Heat the oil over medium heat in a large, heavy soup pot. Add the onion and cook, stirring, until tender, about 5 minutes. Add the grated potato, the shredded cabbage and 1/2 teaspoon salt. Stir together for a minute, taking care that the potatoes don’t stick to the pot, and add the water or stock, the Parmesan rind, and salt and pepper to taste. Bring to a simmer, cover and simmer over low heat for 30 minutes, until the vegetables are tender. Add the milk to the soup. Stir to combine well and heat through without boiling. A handful at a time, stir the Gruyère into the soup and continue to stir until the cheese has melted. Taste and adjust seasonings. Remove the Parmesan rind. Serve, garnishing each bowl with a handful or toasted croutons and a sprinkling of minced chives.