Prep the crockpot base: Add the diced onion, frozen vegetables, condensed soup, chicken broth, garlic powder, onion powder, thyme, parsley, pepper, and salt to the slow cooker. Stir to combine.
Add the chicken: Place the chicken breasts or thighs into the mixture, nestling them down so they’re mostly covered.
Cook: Cover and cook on Low for 5–6 hours or on High for 2.5–3.5 hours, until the chicken is tender and easily shreds.
Avoid opening the lid often.
Shred the chicken: Remove the chicken to a cutting board and shred it with two forks. Return the shredded chicken to the crockpot. Stir in the heavy cream if using.
Taste and adjust salt and pepper.
Prep the biscuits: Open the biscuit can. Cut each biscuit into quarters for quicker, more even cooking.
Add the biscuits: Scatter the biscuit pieces evenly over the top of the chicken mixture. Gently press them just onto the surface, but don’t fully submerge.
Finish cooking: Cover and cook on High for 1–1.5 hours, or until the biscuits are puffed and cooked through in the center.
Rotate the lid once or twice to reduce condensation dripping onto the biscuits.
Optional finishing touch: Melt the butter and brush it over the tops of the biscuits. Let the casserole rest for 5–10 minutes to thicken slightly before serving.