Cut potatoes into 1 1/2 inch cubes. Steam for 25 minutes or until potatoes are tender. Heat 1 tablespoon walnut oil in a medium skillet. Add ham, walnuts, and garlic; sauté for 10 minutes. Add vinegar and stir well. Toss potatoes, ham mixture, remaining 2 tablespoons walnut oil, bell pepper, green onions, salt and pepper in a large bowl. Serve warm or cold.