Slice the white onions into half moons.
Place the chicken breasts in the base of the slow cooker and sprinkle the onion slices over and around the chicken. Season with salt and pepper. Pour the beef stock around the chicken and sprinkle the Italian seasonings over the top.
Set on low and cook for 8 hours. With 30 minutes, left in the cooking cycle, swirl the cornstarch with a few Tbsp of water and stir it into the sauce. Cook for 30 more minutes to thicken the beef and onion sauce. Add chicken and onions onto plate and top with the grated Parmesan cheese and sliced Swiss cheese. Heat from the chicken should melt the cheese, but if it doesnt you can bake it for a few minutes in oven-safe dish to melt the cheese.
Place thin slices of French bread onto baking sheets and add pinchful of shredded mozzarella onto each bread. Bake at 350 F for 8 minutes, or until cheese has melted. Serve cheese covered French bread slices as side dish, or scoop for the chicken and onions. (Omit if GF.)
Serve Slow Cooker French Onion Chicken with cheesy bread and veggies.