Irish Colcannon with Cabbage and Leeks

Name: Irish Colcannon with Cabbage and Leeks
Ingredients: 2 lbs Yukon gold potatoes (or other medium-starch potatoes); 1/2 tablespoon extra virgin olive oil; 2 cups green cabbage, finely shredded; shreds cut into bite-size pieces; 1 leek (white and light green part only), cut in half lengthwise and sliced into fine half-rings; 2 cloves garlic, crushed and finely minced; 2 tablespoon unsalted butter; 1/2 cup sour cream; 3/4 cup milk (I used skim, but you can use whatever kind you like); 1/2 teaspoon salt, or to taste; 1/2 teaspoon freshly ground black pepper, or to taste; 1/2 cup scallions, thinly sliced (for serving); 2 tablespoon Italian parsley, finely chopped (for serving)