Use a food processor to finely dice/puree the onion, carrot and celery (you may have to do these separately depending on the size of your food processor).
Place the pureed veggies into the crock pot along with the tomatoes chicken broth.
Put the lid on and allow to cook for 7 hours on low.
In a large skillet, melt the butter on medium heat.
Whisk in the flour until well combined, and continue to whisk for 1 minute.
Take out about 1.5 cups of the hot stock out of the crock pot and slowly, 1/4 cup of a time, whisk into the flour in the skillet. Whisk one addition completely in before adding the next 1/4 cup.
When all the stock has been added, allow to cook for 1-2 minutes more.
Transfer this into the crock pot and add the cheese tortellini with the oregano and salt and pepper.
Place the lid on and allow to cook for about 15 minutes more.
Stir in the fresh chopped basil before serving.
Top with fresh grated parmesan cheese or shredded italian cheeses.