Trim any excess fat from the roast and season generously with salt and pepper. Let it sit at room temperature for at least 30 minutes before cooking.

In a large, heavy skillet or Dutch oven, heat the butter over medium-high heat. Once hot, add the beef roast and sear it on all sides until deeply browned, about 10 minutes total.

Remove the roast from the pan and set aside. In the same pan, add the sliced onions and a pinch of salt. Reduce the heat to medium-low and cook the onions slowly, stirring occasionally, for about 20-25 minutes until they are caramelized and golden.

Once the onions are nicely caramelized, sprinkle flour over them and stir. Add the beef broth, scraping up any brown bits stuck to the bottom of the pan.

Return the seared roast to the pan, spooning some onions and sauce over the top. Cover tightly and braise in a preheated oven at 350°F (175°C) for about 2 hours or until the beef is fork-tender.

Remove the roast from the oven and let it rest for about 10 minutes before slicing. Serve slices drizzled with the onion gravy and sprinkle with fresh parsley for a touch of color.