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 mixture.
03 - In a mixing bowl, whisk together the balsamic vinegar, chicken broth, honey, olive oil, Dijon mustard, dried basil, dried oregano, kosher salt, and black pepper until fully incorporated and smooth.
04 - Pour the balsamic glaze evenly over the chicken breasts, ensuring each piece is well coated. Cover with the lid and cook on HIGH for 4 hours or on LOW for 6 to 8 hours, until the chicken is fork-tender and reaches an internal temperature of 165°F.
05 - Carefully transfer the chicken breasts and onions from the slow cooker to a serving platter, tent loosely with foil, and let rest for 5 minutes.
06 - For a thicker, more concentrated sauce, pour the remaining cooking liquid into a small saucepan and simmer over medium-high heat for 3 to 5 minutes until it reaches the desired consistency.
07 - Spoon the onions and reduced sauce over each chicken breast. Finish with a sprinkle of fresh chopped parsley and serve immediately alongside mashed potatoes, steamed rice, or roasted vegetables.