Heat olive oil in a 6-quart stock pot over medium heat. Add onion and garlic, and saute 5 minutes until softened.
Add tomatoes and liquid, chicken broth, and rosemary. Bring to a boil then reduce heat, and simmer, covered, 5 minutes.
Add tortellini, return to a low boil, and cook until pasta is al dente, approximately 6 minutes.
Remove from heat. Stir in chopped spinach. Let stand approximately 2 minutes, until spinach is wilted. Season to taste with salt and pepper. Serve with a spoonful of pesto or pass Parmesan cheese.