Bring a large pot of salted water to a boil. Cook the tortellini according to package directions, then drain and set aside.
Heat 1-2 tablespoons of olive oil in a large pot or Dutch oven set over medium heat. Add the chopped onion and sauté for about 5 minutes. As the onion softens, chop the carrots and celery, then add them to the pot with onions. Mix well and cook for an additional 5 minutes.
Add the minced garlic and butter. Mix well. Once the butter is melted, sprinkle the veggies with ¼ of all-purpose flour. Mix, stirring continuously, for approximately one minute to cook out the flour.
Slowly add the chicken broth, whisking continuously.
Add the dried thyme, dried oregano, Italian seasoning, salt, and pepper. Add the milk and heavy cream.
Increase heat to high. Once the soup almost reaches a boil, immediately reduce heat to medium-low.
Add the cooked, shredded chicken and frozen corn. Cook until heated and the soup has returned to a simmer.
Add the precooked tortellini to the simmering pot or add the cooked tortellini to individual soup bowls.
Serve garnished with freshly chopped parsley, thyme, or grated parmesan cheese, if desired.