Preheat the oven to 275°F. Pat the beef dry. Slice the onion and smash the garlic. Season the beef all over with the kosher salt and black pepper.
Heat the neutral oil in a heavy skillet or roasting pan over medium-high heat. Sear the beef on all sides until well browned, 8–10 minutes total.
Transfer the beef to a rack set in a roasting pan and roast until the center registers 125°F for medium-rare, 45–60 minutes. Move the beef to a cutting board, tent loosely with foil, and rest 20 minutes; for paper-thin slices, chill 15 minutes after resting.
While the beef roasts, make the au jus: Set the searing pan over medium heat. Add the sliced onion and smashed garlic and cook, stirring, until softened and lightly browned, 5–7 minutes. Add the thyme and bay leaf and cook until fragrant, 30 seconds. Pour in the beef stock, scraping up browned bits, then add the Worcestershire sauce. Bring to a simmer and cook until flavors meld and the liquid reduces slightly, 25–30 minutes. Strain into a saucepan and stir in the fine sea salt; keep hot over low heat.
Melt the unsalted butter. Split the French sandwich rolls and brush the cut sides with the melted butter. Toast cut-side up under a broiler or on a hot skillet until lightly golden, 1–2 minutes.
Thinly slice the rested beef across the grain. Pile the slices evenly onto the toasted rolls, spooning a little hot jus over the meat to moisten, then close the sandwiches.
Ladle the remaining hot au jus into small cups and serve alongside for dipping. Serve immediately.