Preheat oven to 375°. In a large frying pan, melt butter over high heat. Stir in flour, remove from heat, and mix to form a smooth paste.
Scrape flour paste into a blender. Add tomatillos, onion, chiles, chopped cilantro, chicken broth, and 1/2 cup sour cream. Whirl until smoothly puréed. Pour green sauce back into frying pan; add salt and lime juice to taste.
Rinse fish, pat dry with paper towels, cut into 6 equal pieces, and season with salt. Set pieces slightly apart in an 8- by 12-in. baking dish.
Bring green sauce to a boil over high heat, stirring. Pour evenly over fish.
Bake fish until it flakes but still looks moist in the center of the thickest part (cut to test), 15 to 20 minutes.
With a wide spatula, transfer fish to plates. Spoon sauce over portions, top with small spoonfuls of remaining sour cream, and sprinkle with cilantro leaves.
How Hot Is Your Chile? To assess a chile's heat, slice off its top through the ribs and seeds, where the heat-producing compound capsaicin is concentrated. Touch the slice to your tongue. If you want your food to be milder, split the chile and scrape out all or some of the ribs and seeds. If your skin is sensitive, wear kitchen gloves or hold the chiles with a fork—and don't touch your eyes.
Note: Nutritional analysis is per serving.