Fill a large pot of water to boil for the pasta.
Heat olive oil in a large pan over medium heat.
While the oil heats, add chopped onion and garlic to the oil. Salt lightly, and stir consistently.
While the onion and garlic are browning, chop the tomatoes.
Add the tomatoes into the pan with the onion and garlic, and toss thoroughly. Cover and simmer on low for 10-15 minutes, or until they begin to break apart.
Add the chopped basil to the tomato sauce, and continue simmering until desired consistency.
Add the canned tuna with its olive oil to the pan. Toss thoroughly. Cover again and warm through.
Towards the end of cooking, add handfuls of baby spinach and toss to help wilt.
Add the cooked pasta directly to the sauce, and toss in the pan so it is thoroughly coated and combined.
Serve with grated parmesan cheese. Enjoy!