Add reserved merguez sausage. Season with salt as needed. Add 1½ cups water and pasta to the same pot. Bring to a simmer while stirring. Cook at a simmer for 3 minutes. Stir in half the mozzarella.

Preheat the oven to 400°. In a large ovenproof skillet, heat the olive oil. Add the merguez and cook over moderately high heat, turning occasionally, until browned all over, about 5 minutes.