Melt butter in a large stockpot over medium heat. Sauté onions, mushrooms, and celery in butter until tender. Add chicken stock and potatoes; simmer for 10 minutes. Add fish; simmer for another 10 minutes.

Mix together clam juice and flour in a small bowl until smooth; stir into soup and simmer for 1 minute. Season with Old Bay, salt, and pepper. Remove from heat and stir in evaporated milk.

Portion into soup bowls and top with crumbled bacon.