- Pat roast dry with a paper towel and rub with garlic halves. Chop garlic halves finely and set aside.
- Season roast with salt on all sides and refrigerate uncovered for 2 to 4 hours, turning once. Bring to room temperature for 30 minutes before cooking.
- In a skillet, brown roast on all sides over high heat.
- Preheat oven to 150°C (300°F). Brush meat with mustard. Set aside.
- In the same skillet, sauté vegetables in oil for about 5 to 6 minutes. Deglaze with vinegar.
- Place vegetables, reserved garlic, thyme, and flour in the bottom of an oven-proof dish and mix to combine.
- Place roast over vegetables.
- Bake for about 1 hour–1 hour 15 minutes, or until internal temperature in roast’s thickest part reaches 63°C (145°F) for rare doneness.
- Check doneness by cutting roast during cooking. Let meat stand in a dish covered with aluminum foil for 10 minutes.
- Using a spatula, transfer vegetables and cooking juices to a saucepan.
- Add broth and bring to a boil. Reduce heat and simmer for 5 minutes.
- Remove thyme sprigs and purée gravy with an immersion blender while adding butter.
- Strain gravy if desired. Taste and adjust seasoning as needed. Heat plates for a few minutes in warm oven (with heat turned off).
- Serve roast quickly with your choice of mashed potatoes, a blend of mashed potatoes and celery root, or sautéed mushrooms.
If beef broth is a little bland, reduce to half and add some veggies, garlic, and herbs.
Roast tastes best served rare.