2 scallions, white parts finely chopped and greens reserved for another use
1 tablespoon unsalted butter
1 to 2 tablespoons finely chopped fresh jalapeņo including seeds
1/2 cup heavy cream
1/2 teaspoon salt
4 (6- to 8-oz) pieces flounder fillet
1/8 teaspoon black pepper
1/2 cup quartered grape tomatoes
1 tablespoon chopped fresh cilantro
Cook chopped scallions in butter in a 10-inch heavy skillet over moderate heat, stirring, until softened, 3 to 4 minutes. Add jalapeņo (to taste), cream, and 1/4 teaspoon salt and bring to a simmer.
Pat fish dry, then sprinkle with pepper and remaining 1/4 teaspoon salt. Fold each fillet in half crosswise. Put folded fish in sauce in skillet and cook at a bare simmer, covered, until fish is just cooked through, 5 to 6 minutes. Transfer fish to a platter and keep warm, covered with foil.
Add tomatoes and cilantro to sauce in skillet, along with any fish juices on platter, and cook over moderately low heat until heated through, about 1 minute. Spoon sauce over fish.