Next, cut both ends off the onions and cut them in half from the top to the bottom. Lay each half flat side down and cut across the rounded tops to create 1/4″ thick half-moon slices.
When the onions are done, add the thyme and flour. Stir to combine the flour and make sure there are no lumps before adding the white wine and beef broth, stirring as you add the liquid.
Top the chicken breasts with the grated/shredded gruyere. Place the Swiss cheese on top of the chicken. Bake in the oven to melt the cheese and it’s bubbling, and the chicken is done in the center, 165ºF.