FOR MUSHROOM SAUCE: Place butter in a heavy skillet. When it is sizzling, add the onion and, a few minutes later, the celery.
When they are slightly browned, add the mushrooms. Reduce heat and allow everything to simmer. When mushrooms are limp, add the wine and season with salt and pepper.
Now combine the flour with milk and add gradually to the mushroom mixture, stirring well. When sauce is thickened, remove from heat. This sauce may be cooled and refrigerated for a week. Then it can be warmed up in a double boiler.
FOR ROAST: Preheat oven to 425°F. Dry the roast on paper towels. Rub the roast all over with olive oil, and season generously with coarse salt and freshly ground pepper. Roast for 10 to 12 minutes. Check the internal temperature with an instant-read thermometer. Roast a few additional minutes, if necessary.
For medium-rare (130–135°F), remove from oven at 125–130°F and let rest for 10 minutes before carving. (The internal temperature will continue to climb 5 to 7 degrees.)
Transfer to a warmed serving platter. Serve with Mushroom Sauce.