Heat a large skillet or cast iron pan over medium-high heat. Sear chorizo slices until nicely browned and fragrant on both sides. Remove and set aside.
In a large pot or Dutch oven, combine the wine, water butter, salt, pepper and saffron over medium heat until simmering.
Add the onion, bell pepper, tomatoes and garlic to the pot, then top with raw mussels, cover. Keep covered for 4-5 minutes or until mussels have opened. Add the browned chorizo back to the top and cover for another minute.
Transfer to the mussels and broth to a large bowl for serving, spoon the sauce over the mussels as you transfer. Serve with fresh bread.
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