Slice the onions into thin slices. If your chicken breasts are thick, cut them in half through the middle. You want 4 pieces of chicken that are about 1" thick. Combine the spices to make the seasoning blend.
Turn the Ninja Foodi on high sear/sauté. Add in the oil and butter and allow to it to heat up. Sprinkle the seasoning blend on both sides of the chicken. Only use about ½ of the seasoning blend in this step, the other half is used for seasoning the onions.
When the oil and butter is sizzling, place the chicken breasts into the inner pot and sear them for 3-4 minutes. Flip the chicken and add the onions. Sear the chicken for another 3-4 minutes.
Remove the chicken breasts and place them on a plate. Stir the seasoning into the onions.
Lower the Tendercrisp lid and select broil for 10 minutes. This will quickly brown up your onions. You can also use the sear/sauté function on high, but you will need to stir the onions occasionally to prevent burning.
MIx 1 tablespoon of cornstarch and add 2 tablespoon of the beef broth and stir to combine.
When the onions are finished caramelizing, turn the sear/sauté on high and deglaze the pan with the remaining beef stock. Make sure to scrape the bottom to release any browned bits (fond) because there is tons of flavor there.
Add the cornstarch slurry and stir. Add the chicken breasts back into the pot and bring the liquid to a boil for 2-3 minutes. Reduce the heat to medium and simmer for about 3-5 minutes.
Cover each piece of chicken with Swiss cheese. Place the rack in the high position and place French bread on the rack. Broil for 5-10 minutes or until the bread is toasted and the cheese is melted.
Place a chicken breast on a piece of French Bread (toasted), cover with onions and the sauce. Enjoy!