Step 1Cook the onions in the butter in a large skillet over medium-low heat until browned, 15 to 20 minutes. Add the apples and cook until softened, 5 minutes. Deglaze the pan with the wine and Calvados or Brandy, scraping up any brown bits. Add the chicken and beef stocks, the water and the thyme; bring to a boil. Add the spice sachet and simmer on low heat for 30 minutes.
Step 2Add the apple concentrate and balsamic vinegar and season to taste with salt and pepper. Remove the sachet.
Step 3To serve, divide the soup between two heavy, oven-proof soup crocks. Top each with croutons and a slice of cheese. Brown under the broiler a couple of minutes before serving.