Preheat oven to 325°F. Line a shallow roasting pan with foil. Place a rack on top of the foil.
Using a pastry brush, brush 6 tablespoons mustard all over the top round roast.
In a small bowl, combine the thyme, salt and pepper; mix well. Sprinkle seasoning completely over the roast.
Arrange roast on prepared roasting pan. Bake, 1-¼ to 1-¾ hours or until an instant-read thermometer inserted horizontally in the thickest part of the roast registers 145°F for medium-rare or 160°F for medium (see tip).
Remove roast from oven and tent with foil. Let rest 5 to 10 minutes.
Meanwhile in a small skillet over medium-low heat, heat oil until shimmering. Add shallots and cook, stirring often, 3 minutes. Add the thyme and cook, stirring 2 minutes or until shallots are softened and golden. Set aside.
In a medium bowl, combine sour cream and mustard. Stir in shallots.
Slice roast across the grain into thin slices. Season to taste with salt.
Arrange beef slices on a platter. Serve with mustard mixture 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 and 160°F for medium.
10 medium shallots will yield approximately ⅔ cup minced shallots. If you end up with slightly more or less, that will be fine in this recipe.
Fresh uncooked top round roast can be refrigerated for up to 5 days and frozen for 4 to 12 months.
Cooked top round roast can be refrigerated 2 to 3 days and frozen for 2 to 3 months.