Black Cod with Olives and Potatoes in Parchment

Name: Black Cod with Olives and Potatoes in Parchment
Ingredients: 1/2 pound small Yukon Gold potatoes; 6 tablespoons extra-virgin olive oil, divided; 1 tablespoon plus 2 teaspoons finely chopped oregano, divided; 2 1/4 teaspoons fine sea salt, divided; 8 (5-ounce) pieces skinless black cod, Pacific cod, or haddock fillet (about 1 inch thick), any bones removed; 1 lemon, very thinly sliced; 6 garlic cloves, thinly sliced; 1/2 cup Kalamata-style black olives, pitted and cut into slivers; 1/2 cup flat-leaf parsley leaves; Equipment: an adjustable-blade slicer; 8 (12- to 15-inch) squares of parchment paper; kitchen string