Heat the butter in a large pan over medium-high heat. Add mushrooms, red onion, garlic, salt, and pepper and cook, stirring, until lightly browned, about 5 minutes. Add the broth and wine, raise the heat to high, and continue cooking until reduced by half, about 4 more minutes.
Remove from the heat, pour 1/2 the soup into a tall plastic container cup, and puree using an immersion blender. Return the puree to the pan and heat over high heat, while stirring in the spinach and cream in batches.
Divide the soup among 2 bowls and serve each garnished with 6 enoki stems.