Roast
Combine the rosemary, salt and pepper in a small bowl and rub over the entire surface of the steak. Set the steak on a sheet pan and let stand at room temperature for 30 minutes.
Preheat the oven to 250°F.
Transfer the sheet pan to the oven and bake until an instant read thermometer inserted into the thickest part of the steak registers 120°F, about 65-75 minutes.
While the steak bakes make the sauce.
Once the steak temperature reaches 120°F (for medium rare, 125°F for medium) remove it from the oven and let it rest 5 minutes.
Heat a large cast iron skillet over high heat until very hot. Add the steak and sear until well browned, 1 1/2 to 2 minutes per side.
Transfer the steak to a cutting board and let stand 5 to 10 minutes before slicing. Serve with the sauce spooned over the slices.
Sauce
Melt 1 tablespoon of the butter in a large skillet over medium heat. Add the shallots and cook, stirring occasionally, until starting to soften, about 2 minutes.
Pour in the wine, stock, rosemary sprig and sugar. Increase the heat to medium-high; bring to a boil and cook, stirring occasionally, until reduced by about two-thirds, about 12-13 minutes.
Discard the rosemary sprig. Stir in the mustard and cook 1 minute longer. Remove from the heat and swirl in the remaining 2 tablespoons butter and salt; keep warm.
