Add oil to a large, deep pot or skillet and place over medium heat.
Add onion and bell peppers and cook until soft, about 4-5 minutes.
Add garlic and cook until fragrant, about 1-2 minutes.
Stir tomato paste into the skillet, using the back of a spoon to combine it with the other ingredients. Cook until caramelized, about 4-5 minutes.
Crumble in the chorizo and cook until browned, about 5 minutes.
Pour in the diced tomatoes and chicken broth and stir to combine. Bring to a boil, then season with salt and pepper.
Add the cleaned mussels and clams to the pot and cover with a lid. Steam for 5-10 minutes, or until all the shells have opened. If any shells remain closed, remove them from the skillet and discard.
Sprinkle fresh cilantro on top, if using, and serve right away.