Coat the beef with oil and sprinkle the salt, pepper, and rosemary evenly all over.
Heat a cast iron or oven-proof skillet on the stove top over medium high heat. Sear the beef on all sides. Transfer the beef in the pan to the oven and roast the beef 15 minutes per pound for rare and 20 per pound for medium.
When the beef is done, remove from the roasting pan onto a platter and cover with foil and allow to rest on a cutting board while you cook the potatoes and gravy.
Add the potatoes to a mixing bowl. Drizzle with vegetable oil, salt and pepper and coat well.
Add the potatoes to the same roasting pan that you removed the beef from , cut-side down and roast in the oven for 45 minutes until golden brown and tender when a knife is inserted.
While the potatoes roast, to a pan over medium heat and melt the butter. Add the onions and cook until they are softened. Stir in the flour and cook until it starts to turn a golden color. Whisk in the beef stock until the flour is incorporated and there are no lump. Simmer until thickened, season with salt and pepper to taste. You can strain the gravy if you wish to remove the onions or any lumps of flour that you can’t remove.
Slice the beef and serve with the potatoes and your favorite vegetable.