Set instant pot to saute mode. Place diced onion, garlic, carrots, and celery into the pot. Saute for 3 minutes. Add in chicken. Turn off saute mode.

Fill broth to just over the chicken mixture in the pot. Do not fill all the way. You just want to cover the vegetables and chicken. Add in rosemary, salt, and pepper. Place lid on the pot. Set valve to sealed position. Set high pressure or manual to 10 minutes. Once the pot stops counting release pressure.

Cook pasta on stove as directed. It's best to cook it separately.

Mix cornstarch and heavy cream together. Whisk well. Add to pot while stirring. Set saute mode back on and let come to a roaring boil for 5 minutes then turn off and let cool some. The cooling process will help the heavy cream and cornstarch thicken.

Drain pasta and lightly stir it into the instant pot chicken and vegetables mixture. Stir in 1/2 cup of sour cream. If you would like it thicker then add a little more sour cream.

Serve topped with fresh rosemary and cracked black pepper.