In a large skillet, melt 2 tablespoons butter over medium heat. Saute onion for about 2 minutes until it starts to get tender. Sprinkle chicken breasts with salt and pepper on both sides and add into the pan, settling the chicken in between the cooking onions. Brown chicken on both sides, not cooking it through. Transfer the chicken and onions to the warming crockpot. Add in mushrooms.
Place large skillet back over medium high heat and melt remaining 2 tablespoons butter. Whisk in flour and cook 1 minute. Slowly whisk in white wine and milk and stir until it simmers gently and thickens slightly. Stir in cream, thyme and any salt and pepper if desired. Pour sauce over chicken, onions and mushrooms and stir. Reduce crockpot to low and cook 3-4 hours or until chicken is tender.
Remove chicken from crockpot and shred. Place back into the hot sauce. Stir in frozen peas, cooked spaghetti and parmesan cheese. Toss and serve. Garnish with parsley if desired.
Optional topping: Melt 1/4 cup butter in a small skillet. Stir in 1/2 cup plain bread crumbs and toast lightly. Sprinkle on top of completed dish as desired.