Step 1

In a small bowl, mix the 2 tsp. cornstarch with the 2 Tbs. vinegar. Add the broth, soy sauce, and ketchup. Season the fish with 1/2 tsp. salt and 1/2 tsp. black pepper. Spread the remaining 1/2 cup of the cornstarch on a plate, and put the eggs in a wide bowl. Dredge the fish in the cornstarch. Heat 2 Tbs. of the oil in a 12-inch nonstick pan over medium-high heat until shimmering hot. Soak the fish in the egg for a couple of seconds, then add to the pan. Cook, flipping after 3 min., until the fish is browned and a little firm to the touch, about 5 minutes total (it should not be cooked all the way through). Transfer the fish to a large plate.

Step 2

Add the remaining 1 Tbs. oil to the pan, then add the chiles, white parts of the scallions, and ginger. Cook, stirring, until the scallions become translucent and browned in places, about 2 min. Give the cornstarch mixture a quick stir and then pour it into the pan. It should come to a boil and thicken quickly. Reduce the heat to medium-low, return the fish to the pan, and flip it a couple of times to coat it in the sauce. Cover the pan, leaving the lid slightly ajar, and cook until the fish is just cooked through and starting to flake, 3 to 5 min. Taste the sauce for salt and vinegar and add more to taste. Serve immediately with the sauce spooned over the fish and sprinkled with scallion greens.