Prep Time: 10 Minutes
Cook Time: 20 Minutes
Total Time: 30 Minutes
1. Pat steaks dry with paper towel and season with salt and pepper. Heat oil in cast-iron skillet over high heat until oil shimmers. Place steaks in skillet and cook to desired degree of doneness, turning once during cooking time. Remove to plate and cover to keep warm while making sauce.
2. Add onion to skillet and cook briefly, stirring onion to keep it from burning. Add vinegar and reduce briefly (stand back as you add this; it can really make you tear up). Add wine and scrape bottom of skillet, getting up any good, browned bits from the skillet. Bring to boil. Reduce slightly and let wine reduce to about 1/3 cup.
3. Remove skillet from heat, add butter and swirl into red wine, whisking if necessary, to incorporate butter into wine, making wine sauce.
4. Pour over steaks and serve immediately.