In a large mixing bowl, whisk together the thawed lemonade, garlic, brown sugar, ketchup, Dijon mustard, apple cider vinegar, and red pepper flakes until smooth.

Season the chicken breasts with lemon pepper seasoning and a little salt.

Heat a large pan over medium-high heat and quickly sear the chicken—about 1 minute per side, just enough to get a little color.

Transfer the chicken to your slow cooker and pour the lemonade sauce right over the top.

Cover and cook on low for 6 hours or high for 3 hours, until the chicken is tender.

Once cooked, remove the chicken from the slow cooker and set it aside for a minute.

In a small bowl, whisk together the cornstarch and water to make a slurry.

Stir the cornstarch slurry into the sauce in the slow cooker to thicken it up.

Return the chicken to the slow cooker and coat it in that lemony sauce.

Garnish with fresh lemon zest and chopped parsley, and you're ready to serve!