Heat the oven to 180°C. Cook the shells following pack instructions, but drain when they are still a little al dente as they will cook more in the oven.
Pour the olive oil into a pan and gently cook the garlic (and chili flakes, if using) for 2-3 minutes. Stir in the spinach and cook until wilted. Stir in the ricotta and lemon zest, then season well.
Spread the tomato sauce evenly across the bottom of a baking dish. Use a teaspoon to pack each shell with the spinach and ricotta mixture, then put them in the dish, making sure the shells are evenly spaced apart.
Sprinkle parmesan on top and bake for 20-25 minutes or until cheese is bubbling and golden. Decorate with basil leaves if you want.
