1.Bring stock to the boil over high heat. Remove from heat, cover, and keep warm.
2.Heat oil in a large, heavy-based saucepan over medium heat. Sauté onion, 3-4 minutes, or until soft.
3.Stir in wine and cook, 1-2 minutes, until it evaporates. Add rice and cook, stirring, 1-2 minutes, until the rice grains start to become translucent.
4.Add 1 cup of stock to the rice, stirring often, until the rice absorbs the liquid.
5.Continue adding remaining stock, 1 cup at a time, allowing it to absorb, until the rice is just tender and creamy (about 20 minutes).
6.Stir through tomatoes, tuna, parmesan and all except a handful of baby spinach. Season to taste. Serve topped with reserved spinach.