Prepare all the ingredients. Melt the butter over medium heat and sauté the onion until translucent.

Sprinkle the cornstarch over the onion and stir until dissolved. Add the broth and bring to a boil over medium heat.

Add the milk and cook for 10 minutes, until slightly thickened. Add the spinach.

Once the spinach has softened, let the mixture cool slightly and blend until smooth.

Return the blended soup to the pot and heat over low, stirring constantly. Add the crumbled blue cheese, then the heavy cream. Mix well.

Season with salt, black pepper, and cayenne pepper to taste. Heat gently until ready to serve, but do not let it boil to prevent curdling. Garnish with croutons and a basil or parsley leaf.