4 (1/4- to 1/3-inch-thick) boneless pork chops (2/3 lb total)
2 tablespoons unsalted butter
1 small onion, halved lengthwise and thinly sliced lengthwise
1/4 lb mushrooms (use crimini, button, shiitake or oyster mushrooms), cut into 1/4-inch-thick slices (remove the stems from the shiitakes and oysters)
1/3 cup sour cream
1/4 cup water
2 tablespoons chopped fresh dill
Pat pork dry and season with salt and pepper. Heat 1 tablespoon butter in a 10-inch heavy skillet over high heat until foam subsides, then sauté chops until browned and just cooked through, about 1 1/2 minutes on each side. Transfer chops to a platter and keep warm, covered.
Reduce heat to moderate and add remaining tablespoon butter to skillet. When foam subsides, cook onion and mushrooms, stirring, until tender, about 6 minutes. Reduce heat to low and stir in sour cream, water, any meat juices that have accumulated on platter, dill, and salt and pepper to taste and cook, stirring, until heated through. (Do not let boil.) Spoon sauce over chops.
Serve with rice or noodles and steamed carrots.