Place the ribeye steak on a wire rack lined baking sheet and salt on all sides. Season with pepper to taste. Refrigerate for at least 12 hours or overnight. Note: If in a rush, you can skip this step and just season with salt and pepper after the steak has been removed from the fridge.
Remove the steak from the fridge at least 1 hour before searing.
Pour the wine and add the rosemary sprig to a saucepot and bring to a moderate boil. Reduce by half. Once reduced, remove the rosemary sprig and set sauce aside.
Preheat the oven to 400°F and set the rack to the middle level.
Heat a large cast iron or stainless steel pan to medium-high heat. Add the oil and once shimmering add the steak. Cook the steak until well browned on both sides, flipping every minute (about 3 minutes per side). Make sure to sear the sides of the steak with tongs.
Place the steak on a wire rack-lined baking sheet and into the oven. Roast until the steak reaches 115°F internal temperature or until desired temp is achieved. Place the steak on a cutting board and tent with foil for at least 10 minutes before slicing.
Pour the oil out from the steak pan and heat to medium. Add the butter and shallot. Once the shallot is soft, add the flour and cook for 1-2 minutes or until it darkens a touch. Whisk in the beef stock followed by the reduced red wine and bring to a boil.
Once the sauce thickens and coats the back of a spoon remove it from heat. After thickening there should be about 1 cup of sauce. Whisk in 1 tablespoon of cold butter and season with salt and pepper to taste. If the sauce is too strong or bitter for your liking a pinch of sugar can be added.
Slice the steak and serve on a platter with the red wine sauce on the side. Enjoy!