Place the clams in a large bowl and cover with cold water for 15 to 20 minutes. They are still alive and they will “spit out” any sand or grit from inside their shells when submerged in cold water.
Cook chorizo and onion in a large, heavy-bottomed pot over medium-high heat, stirring often with a wooden spoon, until the chorizo is cooked through and the onions are soft, about 5 minutes. Stir in the garlic and cook for an additional 1 minute.
Stir in the tomatoes, heavy cream, and salt and bring to a simmer. Taste and add more salt if needed.
Drain the clams, then rinse thoroughly.
Stir in the clams and cover the pot. Cook until the clams open, 6 to 10 minutes. Discard any clams that don’t open.
Garnish with fresh herbs and serve with baguette.