Soy sauce, sherry, and thyme added early on helped boost flavor of our Slow-Cooker French Onion Soup recipe, but the addition of beef bones reproduced the rich meatiness of the classic. We microwaved the bones for a few minutes before placing them in the slow cooker to simulate roasting without having to turn on the oven or dirty a roasting pan. Apple butter provided one of our favorite pork chop recipes with a silky sauce, and it had a similar effect in our Slow-Cooker French Onion Soup. Best of all, tasters never knew it was there. Because broiler-safe soup crocks are hard to come by, we opted to broil slices of bread topped with Gruyère cheese and add them to the soup at the end.
After quartering, slice the onions pole to pole for substantial slices that will hold up to long cooking (see pictures below). Beef bones are stocked in the freezer or meat section of supermarkets.
COOK ONIONS Arrange beef bones on paper towel-lined plate. Microwave until well browned, 8 to 10 minutes. Meanwhile, set slow cooker to high. Add butter, cover, and cook until melted. Add onions, 2 teaspoons salt, 1 teaspoon peper, brown sugar, and thyme. Stir flour, apple butter, sherry, and soy sauce together in small bowl until smooth. Pour over onions and toss to coat. Tuck bones under onions around edge of slow cooker. Cover and cook on high heat until onions are softened and deep golden brown, 10 to 12 hours. (Cooked onions can be refrigerated in airtight container for 1 day.)
FINISH SOUP Remove bones from slow cooker. Heat chicken and beef broth in microwave until beginning to boil. Stir into slow cooker. Season with salt and pepper.
MAKE CROUTONS Adjust oven rack to upper-middle position (about 6 inches from broiler element) and heat oven to 400 degrees. Arrange bread slices in single layer on baking sheet and bake until bread is golden at edges, about 10 minutes. Heat broiler. Divide cheese evenly among croutons and broil until melted and bubbly, 3 to 5 minutes.
SERVE SOUP Ladle soup into bowls and top each with 2 croutons. Serve.