Heat olive oil in a large pot over medium- high heat. Add onion and cook for 3-5 minutes, or until tender. Add garlic and cook for an additional minute.
Add diced tomatoes, crushed tomatoes, tomato paste and Italian seasoning and mix until combined. Let cook for 5 minutes, then mix in chicken stock and bring to a low boil. Reduce heat to low and let simmer for 15 minutes.
Add tortellini to the pot and cook for 6-7 minutes, or until soft.
Add parmesan, milk, greek yogurt, spinach and cooked chicken; mix until combined. Cook for a few minutes until heated through and spinach is wilted. Season with salt and pepper, to taste.