For garlic and thyme oil, heat ingredients in a small saucepan over medium heat until fragrant and garlic is light golden (2-3 minutes), then set aside.
Preheat a sandwich press. Drizzle cut sides of ciabatta with a little garlic and thyme oil and spread bases with ’nduja. Top with mozzarella slices, season with sea salt and sandwich with tops. Toast until mozzarella melts (3-4 minutes), then cut into pieces, drizzle with garlic and thyme oil, and serve hot with pickled onions.
Note’Nduja is available from select delicatessens. If it’s unavailable, substitute sliced spicy salami. Soften ’nduja at room temperature to make it easier to spread.