Sheet Pan Fish With Lemon Potatoes

Name: Sheet Pan Fish With Lemon Potatoes
Ingredients: 1 1/2 pounds small Yukon Gold potatoes (not baby potatoes); 1/2 onion, thinly sliced; 1/4 cup extra-virgin olive oil; Kosher salt and freshly ground pepper; 1 tablespoon plus 1 teaspoon finely grated lemon zest, plus 2 tablespoons lemon juice; 4 teaspoons finely chopped fresh oregano; 2 cloves garlic, minced; 4 skin-on branzino fillets (about 1 1/2 pounds); 1/2 cup finely chopped jarred roasted red peppers, plus 2 tablespoons brine; 2 teaspoons capers, roughly chopped; Chopped fresh parsley, for topping