Heat butter and oil in a 12-inch heavy skillet over medium heat until foam subsides. Meanwhile, pat steaks dry and sprinkle both sides with 3/4 teaspoon salt and 1/2 teaspoon pepper (total). Cook steaks, turning once, until browned and medium-rare, 6 to 8 minutes total. While steaks cook, stir soup in can, then transfer 2/3 cup to a blender (reserve remainder for another use) and blend until very smooth, about 2 minutes. Transfer steaks to a cutting board and let stand, loosely covered, 10 minutes. Meanwhile, pour off all but 1 tablespoon fat from skillet and cook scallions over medium heat, stirring, until softened. Stir in blended soup along with remaining ingredients and any juices from meat and bring to a boil. Spoon some of sauce over steaks and serve remainder on the side.