In a large soup pot over medium-high heat, add the oil. Add the red pepper flakes and chorizo. Cook for 1 to 2 minutes to release the flavors. Stir in the onions and fennel and cook until they caramelize, about additional 2 to 3 minutes. Add the garlic and saffron and allow the saffron to bloom. Stir in the wine and stock and mix well. Add the mussels and stir. Cover the pot and steam until the mussels open, about to 5 minutes. Remove the lid, then add the tomatoes, herbs, lemon juice and salt and pepper, and stir well. Ladle the mussels and broth into large serving bowl and serve with crusty bread.