Combine green onion, butter, and 1/4 teaspoon seafood seasoning in a saucepan over medium heat. Cook until onion is slightly softened, 2 minutes.
Stir tomato soup into saucepan; increase heat to medium and bring to a simmer. Stir in cayenne pepper and reduce heat to medium-low. Stir in crabmeat; cook, stirring occasionally, until crab is cooked through, 5 to 10 minutes. Season with salt and black pepper to taste.
Remove from heat.
Whisk cream with a pinch of seafood seasoning until light and frothy, about 30 seconds. Ladle bisque into bowls and stir in a teaspoon of frothy cream.