1In a Dutch oven or other large pot, melt 3 Tbsp. unsalted butter on the stovetop over medium heat. Add 1 Tbsp. vegetable oil and 3 lbs. Vidalia onions (about 4 medium), halved lengthwise, peeled, and thinly sliced, and cook until onions are softened, stirring occasionally, about 15 minutes. Add 1 tsp. Diamond Crystal or ½ tsp. Morton kosher salt, ½ tsp. freshly ground black pepper, and ½ tsp. granulated sugar. Continue to cook, stirring occasionally, until onions are deep brown, almost mahogany in color, and caramelized, reducing heat slightly if onions seem to be browning too quickly, 45 minutes to 1 hour more. (If the bottom of your pot begins to burn at any time during the cooking process, add 1–2 Tbsp. water and scrape the bottom, stirring in the browned bits.) Do ahead: Caramelized onions can be prepared 1 week ahead; refrigerate in an airtight container or freeze for up to 6 months.
3Tie 10 sprigs fresh thyme and 2 dried bay leaves into a bundle with twine. Add 6 cups Beef Bone Broth or store-bought low-sodium beef stock and herb bundle to pot with onions. Bring to a boil, reduce to a simmer, and cook, uncovered, until broth is thickened and flavorful, 20–30 minutes. Remove from heat and whisk in remaining 2 Tbsp. unsalted butter. Taste and adjust seasoning with additional kosher salt and freshly ground black pepper. Do ahead: Soup can be made 3 days ahead; refrigerate in an airtight container or freeze for up to 6 months. Reheat over medium until steaming.
