How to Make It
Preheat oven to 425°F.
Combine potatoes, 1 tablespoon oil, 1/4 teaspoon salt, and 1/4 teaspoon black pepper in a bowl; toss to coat. Spread potatoes in a single layer on an aluminum foil–lined baking sheet. Roast at 425°F until tender, 20 to 25 minutes, stirring after 15 minutes.
Stir together onion, parsley, vinegar, oregano, red pepper, 3 tablespoons oil, and 1/4 teaspoon salt. Set chimichurri aside.
Heat a large cast-iron skillet or grill pan coated with cooking spray over high. Rub salmon with remaining 1 tablespoon oil. Sprinkle with remaining 1/2 teaspoon salt and 1/2 teaspoon black pepper. Add salmon to pan, skin side down; gently press until skin flattens, about 15 seconds. Cook until skin is browned, about 4 minutes. Flip salmon; cook to desired degree of doneness, about 2 minutes for medium-rare. Serve with chimichurri and potatoes.