To prepare the mussels, first wash them well. Put them in a big bowl and fill it with cold water. Repeat this process a few times, stirring the mussels in the water and draining it. Scrub the shells and remove the beard if needed.
In a large pot or pan with a lid, heat 1 tablespoon of olive oil over medium-high heat.
In a medium skillet, heat the remaining olive oil over medium heat. Add the onion and cook until soft and translucent, stirring frequently, for about 8 minutes. Season with salt, then stir in the the garlic and the chili. Cook for another minute or two until the garlic is soft.
Stir in the tomatoes, a pinch of sugar, and salt and pepper to taste. Bring the sauce to a boil over medium-high heat, then lower the heat and cover the skillet. Simmer for 20 to 30 minutes, stirring occasionally, until the sauce is thick and aromatic.
Pour the white wine and saffron (this is a very expensive spice, so you can skip it if you want) into a pot and bring it to a boil.
Add the mussels, put the lid on and let them steam for 4 to 5 minutes, until they open up. Halfway through, give the mussels a stir to make sure they cook evenly. Use tongs to transfer the opened mussels to a bowl. Throw away any that remain closed.
Add the cilantro to the tomato sauce. Stir well and check the seasoning.
Divide the mussels into 4 large soup bowls and pour the sauce on top. Enjoy with some crusty bread to dip in the sauce.