Celery Root Fennel Soup

Name: Celery Root Fennel Soup
Ingredients: 2 tablespoons butter; 2 cups onions , thinly sliced (sliced across the grain); 4 cloves garlic , sliced; 2 medium fennel bulbs , thinly sliced; 2 medium celery roots (about 1 1/2 pounds ), peeled and thinly sliced (See TheKitchn's guide to peeling celery root ); 1 medium russet potato , peeled and thinly sliced; 2 bay leaves; 7 cups chicken stock; 2 1/2 teaspoons kosher salt (less or more to taste); 1/4 teaspoon black pepper; 1/4 cup whipping cream; Fennel fronds, for garnish