Preheat oven to 375ºF. Toss together the potatoes and onions and set aside. In a large skillet over medium heat, heat the oil and add the ham. Sauté the ham until just heated through, about 2 minutes. Remove from pan and cut into thin strips.

In a small bowl, mix together ricotta cheese, rosemary and black pepper. Place half of the potato-onion mixture in a 13 x 9-inch baking dish and top this with the ricotta cheese and ham strips. Cover with the remaining potatoes. Pour the milk over the entire casserole. Bake for 1 1/2 hours, or until the potatoes are soft and the sauce is thick and bubbly. Top with the grated cheese and bake for 15 to 20 minutes more, or until the cheese melts. Serves 6 to 9.