Preheat Traeger grill to 425F.
Pat chicken very dry, drizzle with 2 tablespoons oil and massage to coat. Liberally season the entire bird with salt, pepper and rosemary. Set aside.
Place whole, unpeeled sweet potatoes on a small sheet of parchment paper. Drizzle with ½ teaspoon oil and season with a small pinch of salt. Wrap tightly to seal then wrap a second time in a small sheet of tin foil. Repeat with remaining sweet potatoes and onions.
Place sweet potatoes and onions towards the very back of the grill where it gets hottest. Lay chicken, breast-side up, in the center of the grill. Close the lid and cook until the thickest part of the thigh registers between 180F and 185F and the thickest part of the breast registers 165F, between 40-45 minutes. Cook the sweet potatoes and onions until they can easily be pierced with a fork, also between 40-45 minutes.
Transfer grilled chicken and vegetables to a serving platter. Unwrap the vegetables, setting half an onion aside for the salsa verde, and let the chicken rest 10 minutes before carving and serving with salsa verde.