1. Heat a 6-qt. pot of salted water to a boil. Add the Brussels sprouts and cook until just tender, 6 minutes. Using a slotted spoon, transfer Brussels sprouts to a bowl of ice water; let sit for 5 minutes. Drain Brussels sprouts and pat dry with paper towels; set aside.
2. Heat 1 tablespoons of the oil in a 12" cast-iron skillet over medium-high heat. Add the chorizo and cook, stirring occasionally, until browned, about 5 minutes. Add the onions and cook, stirring occasionally, until golden brown and soft, about 8 minutes. Add the garlic and cook until soft, about 2 more minutes. Transfer chorizo mixture to a bowl. Increase heat to high and add the remaining oil and the reserved Brussels sprouts; cook, flipping once or twice, until the Brussels sprouts are browned and tender, about 8 minutes. Stir in the reserved chorizo mixture and season with salt and pepper.