Add 3 tablespoons of butter, chicken, onion, carrots and celery to a large pot over medium-high heat. Cover and cook for about 10 minutes. Mix in garlic, salt, pepper, sage, thyme and nutmeg. Cook for 1 minute.
Make a roux to thicken the soup by adding the remaining 3 tablespoons of butter to the pot with the chicken and vegetables and mix until melted. Sprinkle with flour and mix again. Cook for 1 minute.
While mixing, slowly pour in chicken broth then milk. Simmer over medium heat for about 10 to 15 minutes until thickened.
While the soup simmers, bring a large pot of water to a boil, then add the tortellini and cook according to package directions. Drain and set aside.
Add cheddar cheese to soup and mix until combined. Then mix in corn and cooked tortellini. If you anticipate leftovers, keep the tortellini separate from the soup until you're ready to serve, then add one serving of tortellini to each individual bowl and ladle the soup over the tortellini. Store the leftover tortellini and soup separate in the fridge.
Serve soup with buttered crusty bread.