Bring a large pot of salted water to a boil. Add Brussels sprouts and cook until al dente, about 6 minutes. Drain and rinse under cold water. Pat dry and cut in half.
Warm oil in a large skillet over medium heat. Add pancetta and fry until crisp, about 10 minutes. Remove pancetta to a plate with a slotted spoon. Add onion to fat in skillet, reduce heat to medium-low and cook, stirring often, until onion is soft and browned, 15 to 18 minutes. Add Brussels sprouts to skillet and gently stir until heated through, 6 to 8 minutes. Return pancetta to skillet. Season with salt and pepper. Serve warm.
