In a large pot melt butter over medium heat. Add celeriac, leeks, rutabaga, garlic and onion. Cook for 10 minutes stirring occasionally until beginning to soften. Add broth and bring to a boil. Cover and reduce heat to low. Let simmer for 20-25 minutes until vegetables are tender. Use a blender to puree soup until smooth. Stir in the cream. Salt and pepper to taste.