Heat oil in a saucepan over medium heat. Add onion and garlic. Cook, stirring often, for 7 to 8 minutes or until onion is soft. Increase heat to high and add rice. Cook, stirring, for 1 minute. Stir in stock. Bring to a simmer. Reduce heat to low. Cover and cook for 10 minutes. Remove from heat. Stand, covered, for 10 minutes. Transfer to a bowl. Set aside for 10 minutes to cool.
Preheat oven to 190°C Grease and line base of a 6cm deep, 20cm (base) springform pan. Add tuna, tomato, bocconcini, parmesan, spinach and egg to cooled rice mixture. Season with salt and pepper. Mix until well combined.
Press rice mixture into prepared pan. Bake for 40 to 50 minutes or until set and crisp around the edges. Stand in pan for 10 minutes. Run a knife around the edge to loosen pie.