Preheat oven to 350°F if using a dark muffin tin, or 375°F for a light-colored pan. Generously spray a 12-cup muffin tin with nonstick cooking spray.
Flatten each jumbo buttermilk biscuit (regular, not flaky layers) with the palm of your hand and press into each muffin cup, working the dough up the sides to form a cup shape.
In a bowl, stir together thawed vegetables (patted dry so the filling doesn't get watery), cream of chicken soup, diced chicken, thyme, garlic powder, onion powder, salt, and pepper. Rotisserie chicken works great here — canned chicken works in a pinch.
Spoon about 3 tablespoons of filling into each biscuit cup. I use a tablespoon scoop to accurately measure.
Bake 20–24 minutes until edges are golden brown. Let stand in the pan for 5 minutes, then run a knife around each cup and remove.