To a large pot of boiling salted water add the Brussels sprouts and cook for 10 minutes, remove with a slotted spoon and drop into a bowl filled with ice and water, after 5 minutes strain and set aside.
In a large saute pan over a medium flame heat the olive oil. When it begins to smoke, add the chorizo, and cook until it becomes crispy and has rendered some fat. Add the onion and garlic and cook until the onion is translucent, increase the flame to high and add the Brussels sprouts and cook for an additional 3 minutes.
Add the chicken stock and cook for 5 minutes, or until the liquid has reduced to approximately 1/4 cup, add the butter and cook for 3 more minutes, season with salt and pepper and serve in a large bowl.