In a large stockpot, melt 2 tablespoons butter over medium heat. Saute onions, mushrooms and celery in butter until tender.
Add chicken stock and potatoes; simmer for 10 minutes.
Add fish, and simmer another 10 minutes.
Mix together clam juice and flour until smooth; stir into soup and simmer for 1 minute more. Season to taste with Old Bay seasoning, salt, and pepper. Remove from heat, and stir in evaporated milk. Top each bowl with crumbled bacon, if desired.