Place roast on rack in roasting pan. Combine oil, garlic, thyme, salt and pepper; spread all over top and sides of roast. Pour 1-1/2 cups (375 mL) water into pan.
Roast in 500°F (260°C) oven for 30 minutes. Reduce heat to 275°F (140°C); roast until thermometer inserted in centre registers 150°F (65°C) for medium-rare, about 1-1/4 hours, or until desired doneness. Transfer to cutting board. Tent with foil; let stand for 15 minutes before carving half of the roast into slices.
Meanwhile, skim fat from pan juices; using flat whisk, whisk flour into juices. Place pan over medium-high heat; cook, whisking, for 1 minute. Pour in stock; bring to boil, whisking and scraping up any brown bits. Reduce heat and simmer until thickened, 2 minutes. Strain; serve all but 1-1/2 cups (375 mL) with sliced beef.
SHEPHERD'S PIE: Cut remaining roast into large cubes; coarsely chop in food processor and set aside.
In large nonstick skillet, heat oil over medium-high heat; cook onion and thyme until softened and golden, about 5 minutes. Add reserved gravy. Stir in chopped beef; simmer for 2 minutes.
In saucepan of boiling salted water, cover and cook potatoes and carrot until tender, about 8 minutes. Remove carrot; dice and set aside. Drain potatoes and return to dry saucepan; mash together with buttermilk, butter, salt and pepper until smooth.
Scrape beef mixture into 8-inch (2 L) square glass baking dish. Cover with carrots and peas; spread mashed potatoes over top. (Make-ahead: Cover and refrigerate for up to 24 hours; add 20 minutes to baking time.) Bake in 400°F (200°C) oven until filling is bubbly and potatoes are golden, about 40 minutes. 4 servings.
Canadian Living Cooks.