Preheat oven to 400ºF. Rub roast with salt and pepper.  Heat oil in a heavy ovenproof skillet over medium-high heat until shimmering hot. Brown roast on all sides, about 7 minutes.  Transfer to oven and cook until internal temperature reaches 120ºF for medium-rare, about 30 minutes (start checking after 20). Let rest for 15 minutes before serving.

To make sauce, heat oil in a dutch oven or heavy skillet over medium-high heat. When shimmering hot, add mushrooms and 1 teaspoon salt. Cook, stirring occasionally, until mushrooms are well browned and most of the moisture has evaporated, about 10 minutes.  Add minced onion and garlic, and cook until softened, about 5 minutes more (reduce heat if necessary to prevent burning). Stir in flour to coat mushroom mixture, then stir in broth, rosemary, and salt and bring to a simmer. When the mixture starts to thicken, stir in cream. (Sauce can be made ahead by 1-2 days and reheated just before serving.)