Preheat the oven to 200°C.
Cook pasta shells in salted boiling water until al dente, drain, and cool.
Mash ricotta and mix with diced mozzarella, parmesan, salt, and pepper.
Shape the cheese mixture into 16 balls, wrap each in a basil leaf, and place into a pasta shell.
Heat olive oil in a frying pan and sweat the garlic until soft.
Add tomatoes, basil, salt, and pepper, then simmer gently for 25 minutes.
Spread a layer of sauce in an ovenproof dish and arrange the filled shells on top.
Cover with the remaining sauce, sprinkle with parmesan, and top with mozzarella slices.
Cover the dish with foil and bake for 35 minutes.
Remove the foil and bake for another 5 minutes before serving.