Melt butter in a large pot over medium-high heat until foamy, about 1 minute. Stir in onion and garlic; cook and stir until onion has softened and turned translucent, about 5 minutes. Stir in flour and cook, stirring constantly, for 2 minutes.

Pour 4 cups chicken stock into the onion mixture, and bring to a full boil; stir in fiddleheads, carrot, and potato. Reduce heat to medium-low, cover, and simmer until vegetables are tender, about 30 minutes.

Blend soup until smooth with a hand blender. Stir in cream; cook and stir until heated through, about 5 minutes. Taste soup, and stir in Worcestershire sauce and soy sauce. If you prefer a thinner soup, add additional chicken stock up to 2 cups. Season with salt and pepper. Garnish with Cheddar cheese.