Place your potatoes in a large pan and cover with cold water. Add salt, then bring to a boil. Turn down the heat and simmer for 15 mins until your potatoes are tender, being careful not to overcook them. Drain and steam dry for a couple of mins.
Place your olive oil, garlic and rosemary in a large baking dish and heat in the oven for 5 mins. Remove your garlic and rosemary.
Using a potato masher or a fork, lightly press down on the potatoes to smash them individually. Tip into your hot oil, add your salt and garlic powder and stir to coat in the oil. Roast in the oven for 45 mins, or until crispy, turning occasionally to ensure they brown evenly.
Place your tomatoes in a small roasting dish along with your olive oil, salt and pepper and shake to coat in the oil and seasoning. Roast for 15 mins until blistered and soft.
Place your basil, pine nuts, garlic and parmesan in the bowl of a food processor and pulse until finely chopped. Leave the motor running and pour in your olive oil in a steady stream until you have a smooth sauce. Season to taste.
Arrange your potatoes and tomatoes on a serving dish, pouring your tomato juices on too. Tear your mozzarella and arrange on the top, along with a drizzle of pesto and a sprinkle of pine nuts. Serve!