Load the slow cooker: add the onion and potatoes to a 6-quart slow cooker first, then lay the chicken on top; pour in the frozen mixed vegetables.
Mix and pour the sauce: whisk the cream of chicken soup, chicken broth, garlic powder, onion powder, thyme, salt and pepper; pour evenly over everything; put the lid on and leave it on.
Cook low and slow: LOW for 6–7 hours, or HIGH for 3–4 hours, until the chicken reaches 165°F (74°C) and the potatoes are tender.
Shred the chicken with two forks and return it to the filling; stir.
Finish the sauce: stir in the heavy cream (or sour cream), lid back on, rest 10–15 minutes to thicken.
Bake the biscuits in the oven exactly as the package directs, until deep golden, while the filling rests.
Serve: spoon the filling into bowls and top each with a biscuit split in half.