Add the chicken breasts, chicken stock, salt, pepper, garlic powder, onion powder, and dried thyme to a large, deep pot or Dutch oven.
Bring to a gentle boil over medium-high heat. Reduce the heat to medium-low, cover, and simmer until the chicken is cooked through. Fresh chicken breasts will generally take 15-20 minutes, while frozen chicken breasts may take 20-30 minutes, depending on their thickness. The thickest part of the chicken should reach 165°F (74°C).
Remove the chicken from the pot and let it rest for 5 minutes. Cut it into small bite-sized pieces and set aside.
Bring the chicken stock remaining in the pot back to a gentle boil. Add the wholemeal rotini and stir well.
Reduce to a steady simmer and cook the pasta according to the package directions, stirring frequently to prevent it from sticking.
Add the frozen mixed veggies during the final 5-7 minutes of the pasta cooking time. Continue cooking until the pasta is tender and the vegetables are heated through.
Return the diced chicken to the pot and stir to combine. If there is too much liquid remaining, simmer uncovered for a few minutes until some of the excess stock has reduced.
Remove the pot from the heat. Stir in the sour cream until the sauce is smooth and creamy.
Let the chicken and noodles stand for 5 minutes to allow the sauce to thicken slightly, then serve.