Brush roast with oil. In a small bowl, combine 2 tablespoons minced onions, 1 tablespoon salt, the thyme, mushroom powder and granulated garlic. Rub mixture into roast.
Place roast on a wire rack inside a shallow pan. Insert an ovenproof meat thermometer horizontally in the thickest part of the roast. Roast for 15 minutes. Reduce heat to 275°F and cook for 2 hours or until the thermometer registers 135°F for medium-rare, 145°F for medium, and 155°F for medium-well. (See tip)
Remove roast from oven and tent with foil. Let stand 15 minutes.
Pour beef juices from the roasting pan into a 4-cup measuring cup. Add beef broth and enough water to measure 3-½ cups. In a medium saucepan over medium-high heat, add the liquids, remaining dried onions, the Worcestershire sauce and sugar. Bring mixture to a boil. Reduce heat and simmer, stirring often, 5 minutes. Season to taste with pepper and more salt (if needed). Strain the mixture into small serving bowls. Keep warm.
Carve the meat against the grain and arrange on a serving platter. Ladle some of the juices (au jus) over the beef slices.
Serve carved rump roast with the remaining au jus alongside.
The meat will continue to rise in temperature 5 to 10 degrees while it is resting. For this reason, I recommend taking your roast out of the oven before it reaches the ideal temperature. The USDA recommended temperatures for beef are 145°F for medium-rare, 160°F for medium, and 165°F for medium-well.
Make sure to use a beef stock and not beef bouillon as the flavors will not be the same.
You can replace the granulated garlic with 3 cloves of minced garlic or about 1 rounded teaspoon minced garlic.
Make gravy in place of the au jus. Make a slurry by combining 2 tablespoons cold water and 1 tablespoon cornstarch. In step 7 whisk the slurry into the liquid while simmering. Cook, stirring, 5 minutes or until desired thickness.