1. In a skillet, heat oil over medium heat for 30 seconds. Add onions and cook, stirring, until softened, about 3 minutes. Add garlic, rosemary and peppercorns and cook, stirring, for 1 minute. Transfer to slow cooker stoneware. Add broth.

2. Stir in squash and apples. Cover and cook on Low for 8 hours or on High for 4 hours, until squash is tender.

3. Preheat broiler. Working in batches, puree soup in a food processor or blender. (You can also do this in the stoneware using an immersion blender.) Ladle soup into ovenproof bowls. Sprinkle with cheese and broil until cheese melts, about 2 minutes. (You can also do this in a microwave oven, in batches, on High, about 1 minute per batch.) Sprinkle with walnuts, if using.

Make Ahead

This soup can be partially prepared the night before it is cooked. Complete Step 1. Cover and refrigerate for up to 2 days. When you’re ready to cook, continue with the recipe.