Heat olive oil in a large pot over medium heat. Add onions; cook and stir until onions start to turn translucent, 3 to 4 minutes. Add chorizo; cook and stir to render some fat, about 3 minutes. Add garlic and cook until fragrant, about 1 minute.

Pour in wine, increase the heat to high, and bring liquid to a boil. Immediately add clams and distribute evenly. Cover and cook until clams open, about 7 minutes, shaking the pot occasionally by holding the pot handles with potholders or a kitchen towel. It's best not to open the lid and stir.

When clams have opened, remove from the heat. Stir in parsley and butter.

Serve clams and liquid in heated bowls.