Three Cheese Conchiglioni with Tomato Sauce - Conchiglioni ai tre formaggi e salsa al pomodoro

Pasta

• 100 g Parmesan cheese, cut in pieces (2 cm)
• 20 fresh basil leaves, plus extra for serving
• 250 g ricotta cheese
• 1 medium egg
• 1 pinch ground black pepper
• 100 g onions, quartered
• 2 garlic cloves
• 30 g olive oil
• 600 g tinned chopped tomatoes
• 1 tsp sugar
• ¼ tsp ground black pepper
• 2 tsp fine sea salt, plus 1 pinch
• water, for cooking pasta
• 16 conchiglioni rigati (large pasta shells)

Assembly

• 125 g mozzarella, sliced (1 cm)