Preheat oven to 180°C. Lightly coat a large baking dish with non-stick cooking spray.
Cook penne in a large saucepan of lightly salted boiling water until tender, following packet instructions.
Heat oil in a medium non-stick frying pan over medium-low heat. Add garlic. Sauté until golden, about 5 minutes. Add spinach and heat through, about 5 minutes. Transfer to a large bowl. Stir in tomato sauce and ricotta.
Drain penne and add to bowl. Combine well. Transfer to baking dish. Top with mozzarella. Cover with aluminium foil.
Bake for 25 minutes. Remove foil. Bake until lightly golden, about 10 minutes. Garnish with olives.