01 - Spread the thinly sliced onions and minced garlic evenly across the bottom of the slow cooker insert.
02 - Lay the chicken breasts over the onion and garlic bed in a single even layer.
03 - In a mixing bowl, whisk together the balsamic vinegar, chicken broth, honey, olive oil, Dijon mustard, dried basil, dried oregano, salt, and black pepper until fully combined.
04 - Pour the balsamic mixture evenly over the chicken breasts, ensuring they are well coated.
05 - Cover the slow cooker and cook on high for 4 hours or on low for 6 to 8 hours, until the chicken is tender and reaches an internal temperature of 165°F.
06 - Remove the chicken and onions from the slow cooker. For a thicker sauce, transfer the remaining liquid to a saucepan and simmer over medium heat for 3 to 5 minutes until reduced.
07 - Serve each chicken breast topped with onions and the balsamic sauce. Garnish with chopped fresh parsley if desired.