Preheat the oven: Preheat your oven to 425°F.
Spatchcock the chicken: Place the chicken breast side down on a cutting board. Using kitchen shears, cut along both sides of the backbone and remove it. Turn the chicken breast side up and press firmly on the breastbone to flatten. Pat the chicken thoroughly dry with paper towels.
Prepare the vegetables: Add the chopped onion, celery, bell pepper, garlic, potatoes, carrots, rosemary, and thyme to a 9-by-13-inch casserole dish or larger roasting pan. Brush with a little softened butter and lightly season with salt, black pepper, and Mrs. Dash Garlic & Herb Seasoning. Spread evenly across the bottom of the dish.
Combine the seasonings: In a small bowl, mix together the Mrs. Dash Garlic & Herb Seasoning, chicken bouillon powder, garlic powder, onion powder, paprika, salt, dried parsley, and black pepper.
Season the underside: Sprinkle approximately 1 tablespoon of the seasoning mixture evenly over the underside of the chicken. Rub gently to distribute. Turn the chicken breast side up.
Loosen the skin: Carefully slide your fingers underneath the skin covering the breasts and thighs, gently separating it from the meat without tearing.
Make the herb butter: Add the remaining seasoning mixture to the softened unsalted butter. Mix until thoroughly combined and spreadable.
Season the chicken: Spread the seasoned butter evenly underneath the skin of the breasts and thighs. Rub the remaining butter over the entire outside surface of the chicken, including the legs and wings.
Arrange in the pan: Place the chicken breast side up over the prepared vegetables. Pour 1 cup of low-sodium chicken broth into the bottom of the dish around the vegetables.
Roast: Bake uncovered at 425°F until the chicken is golden brown and the thickest parts of the breast and thigh reach an internal temperature of at least 165°F. Begin checking around 60 minutes; total roasting time may be approximately 60–90 minutes. Baste with pan juices every 30–45 minutes if desired.
Rest: Remove the chicken from the oven and allow it to rest for 10–15 minutes before carving. If the vegetables need more time, return them to the oven until tender.
Serve: Carve the chicken and serve with the roasted vegetables and flavorful pan juices. Enjoy!