Brown chicken on both sides in a pan with oil. Remove from heat and let rest until onions are ready.
In a large skillet over medium heat, melt the butter. Add the sliced onions and cook, stirring occasionally, until they are caramelized and golden brown (about 20-30 minutes). You want the onions to be deeply browned for that classic French onion flavor. Note: If you’re short on time, cook the onions the night before.
Add the caramelized onions, minced garlic, all vinegars, beef broth, and all spices to the crock pot. Stir everything together to combine. Nessle chicken just under the liquid near the top.
Cover and cook on low for 8 hours or on high for 5 hours. The chicken should be tender and easily shredded.
About 30 minutes before serving, remove the chicken from the slow cooker, shred it with two forks, and return it to the soup.