Preheat oven to 350°. Pat brisket dry with paper towels; trim fat. Melt butter in a 6-quart Dutch oven over medium-high heat. Sear brisket 6 to 8 minutes until golden brown, turning halfway through. Remove brisket from Dutch oven; set aside.

Add onions to Dutch oven; cook over medium heat 20 to 25 minutes or until onions begin to brown, stirring occasionally. Cover and cook on medium-low heat 20 to 25 more minutes or until onions are golden brown, stirring occasionally.

Stir in sherry and vinegar. Cook and stir over medium-high heat until liquid evaporates. Stir in beef stock and 2 thyme sprigs. Bring to a boil; return brisket to Dutch oven.

Cover and bake 1 hour 45 minutes to 2 hours or until brisket reaches 205°. Remove from oven; uncover and cool brisket in cooking liquid for 45 minutes. Cover and refrigerate overnight for flavors to blend and easier slicing.

To serve, remove and discard thyme sprigs. Slice brisket across the grain; return to the Dutch oven with cooking juices. Cover and heat over medium heat 15 minutes or until heated through. Garnish with additional thyme sprigs and Italian parsley, if desired.