Season your chicken breast generously on both sides with salt and pepper.
In a large skillet, melt ¼ cup of butter over medium-high heat. Add the chicken breast and sear on both sides for 2 to 3 minutes or until slightly browned.
Spray inside slow cooker with non-stick cooking spray. Place your chicken breast in the slow cooker.
Add chicken stock to the skillet and deglaze the bottom of the pan by scraping with a wooden spoon. Add lemon juice and stir gently to combine. Pour the chicken stock from skillet over the chicken in the slow cooker.
Tuck onion wedges around chicken. Top with lemon zest, minced garlic, additional pepper, and any remaining butter, cut into slabs.
Cook on high for 3 to 4 hours or low for 6 hours. Remove the chicken and keep warm. Transfer broth to large skillet and heat it to a boil over medium-high heat. Stir in some parmesan and whisk to blend. Then, add the cream and cook until reduced by half while stirring often.
Serve your chicken with cream sauce, chopped parsley, and/or additional sliced lemon as garnish.