Season chicken breast generously on both sides with salt and pepper.

In a large skillet, melt ¼ cup 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 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 chicken stock from skillet over chicken in slow cooker.

Tuck onion wedges around chicken. Top with lemon zest, minced garlic, additional pepper, and remaining butter, cut into slabs.

Cook on high for 3 to 4 hours or low for 6 hours. Remove chicken and keep warm. Transfer broth to large skillet and heat to boiling over medium-high heat. Stir in parmesan and whisk to blend. Then, add the cream and cook until reduced by half, stirring often.

Serve chicken with cream sauce and chopped parsley and/or additional sliced lemon as garnish.