Fish and Chips with Minty Mushy Peas

Name: Fish and Chips with Minty Mushy Peas
Ingredients: 2 tablespoons unsalted butter; ½ small onion, finely chopped; 2 cups shelled fresh peas (from about 2 pounds pods) or 10 ounces frozen peas, thawed; ¼ cup vegetable stock or water; Kosher salt and freshly ground black pepper; ¼ cup fresh mint leaves; Vegetable oil (for frying; about 5 quarts); 2 large russet potatoes (about 1 pound), peeled, cut into ½-inch-thick sticks; 1 12-ounce can lager beer (such as Budweiser); 1 teaspoon soy sauce; ¼ teaspoon cayenne pepper; 1⅓ cups plus ½ cup self-rising flour; Kosher salt and freshly ground white or black pepper; 4 4-ounce strips haddock or cod; A deep-fry thermometer