In a large pot, heat the olive oil over medium heat. Add the diced onion, garlic, carrot, and celery. Sauté for about 5-7 minutes until the vegetables are soft and the onion is translucent. If using, add the red pepper flakes and cook for an additional minute.

Pour in the white wine and bring to a simmer. Let it cook for about 3-4 minutes to reduce slightly.

Add the fish or seafood stock and bring the mixture to a boil. Reduce the heat and let it simmer for 10 minutes.

Stir in the heavy cream and saffron threads. Season with salt and pepper to taste.

Add the cleaned mussels to the pot. Cover and cook for about 5-7 minutes, or until the mussels have opened. Discard any mussels that do not open.

Serve the soup hot, garnished with fresh parsley and lemon wedges on the side.