Spray the inside of the slow cooker with cooking spray or cooking oil.
Add the sliced onions and mushrooms to the bottom of the pot.
Drizzle the olive oil in a large skillet and over medium heat.
Add the chicken breasts to the skillet, season with salt and pepper.
After about 3 minutes on each side (when browned all sides), add the chicken breasts to the slow cooker, on top of the mushrooms.
Pour the pasta sauce all over the chicken breasts, evenly.
Add the bay leaves and sprinkle with basil and oregano.
Cover and cook on low for 6-7 hours or high for 3-4 hours.
After cooking time is over, sprinkle the cheese evenly on top of the chicken breasts and allow it to sit in the slow cooker for 10 minutes, without the lid, to melt the cheese.
Serve on a bed of spaghetti, garnished with fresh basil.