In a small saucepan set over high heat, combine red wine and shallots in a small saucepan set over high heat. Bring to a boil, and reduce to medium heat. Simmer until reduced by half, 8 to 10 minutes.
Heat reserved skillet, without washing, over high heat. Once hot, remove skillet from heat, and carefully pour in wine mixture; stir with a wooden spoon, scraping up any browned bits. Simmer until sauce is reduced by half. Add salt and any juices that have collected under steak; stir to combine. Remove from heat. Add butter; swirl in pan until incorporated. Pour over steak, and serve immediately.