Step 1: Sauté the Vegetables: In a large pot, heat the olive oil and butter over medium heat. Add the diced onion and sauté for about 5 minutes until translucent. Stir in the minced garlic and cook for another minute until fragrant.
Step 2: Cook the Cauliflower: Add the chopped cauliflower to the pot and pour in the vegetable broth. Bring to a boil, then reduce the heat and simmer for about 15-20 minutes, or until the cauliflower is tender.
Step 3: Blend the Soup: Remove the pot from heat and use an immersion blender to puree the soup until smooth. Alternatively, carefully transfer the soup to a blender in batches and blend until creamy. Return the soup to the pot.
Step 4: Add Milk and Cheese: Stir in the milk and grated cheddar cheese, mixing until the cheese is fully melted and the soup is creamy. Season with salt and pepper to taste. If the soup is too thick, add more broth or milk to reach your desired consistency.
Step 5: Prepare the Crispy Sage: In a small skillet, heat a little olive oil over medium heat. Add the fresh sage leaves and fry for about 1-2 minutes until crispy. Remove from the skillet and drain on paper towels.
Step 6: Toast the Hazelnuts: In the same skillet, add the chopped hazelnuts and toast for about 2-3 minutes until golden and fragrant. Be careful not to burn them.
Step 7: Serve: Ladle the soup into bowls and top with crispy sage and toasted hazelnuts. Garnish with fresh chives if desired. Serve hot with crusty bread or a side salad.