01 - Scatter the thinly sliced onions and minced garlic evenly across the bottom of the slow cooker insert.
02 - Lay the chicken breasts in a single layer on top of the onion and garlic bed.
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 thoroughly 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 - Transfer the chicken and onions to a serving platter. For a thicker sauce, pour the remaining liquid into a saucepan and simmer over medium heat for 3 to 5 minutes until reduced.
07 - Spoon the onions and sauce over the chicken and garnish with fresh chopped parsley if desired.