Preheat oven to 375. Toss potatoes & onions with salad dressing and 1/2 tsp of thyme. Arrange in 15x10x1 baking pan sprayed with cooking spray. Bake for 10 minutes.
Meanwhile, combine stuffing mix with water. Cut tenderloin into 2 inch pieces. Remove pan from oven and push veggies to the edges of pan. Spoon stuffing into center of pan; top with meat.
Sprinkle remaining thyme over meat (I also lightly brushed with some of the dressing).