Preheat the oven to 200°C/fan 180°C/gas mark 6. Add the cauliflower steaks to two large baking trays and drizzle with the olive oil. Season well, making sure they are covered on both sides.
Transfer to the oven and roast for 30 minutes, turning the cauliflower over halfway through. Meanwhile, cut the mozzarella into slices. Place in a single layer on some kitchen towel and cover with more kitchen towel to help absorb any excess moisture.
Preheat your grill to medium. Spread 1 tbsp tomato paste over one side of each cauliflower steak. Add the mozzarella, then top with the pepperoni. Sprinkle over the herbs. Transfer to the grill and cook for 2 minutes until the cheese melts and the pepperoni begins to crisp on the edges. Serve with the salad and cucumber.