Cook pasta according to package directions. Reserve 1 cup pasta water before draining.
Heat olive oil in a large skillet over medium heat.
Add diced onion and cook for 3-4 minutes until softened.
Add garlic and cook for 1 minute.
Add drained tuna and cook for 2 minutes.
Add diced tomatoes and Italian seasoning.
Stir in cooked pasta and 1/2 cup pasta water.
Simmer for 3-4 minutes.
Stir in heavy cream and heat through.
Season with salt and pepper.
Garnish with fresh parsley and serve with Parmesan cheese.