How do I prevent the bottom of the roasting pan, onions, and potatoes from being burned when roasting a whole chicken in the oven?

I have been trying to cook roasted chicken in the oven as per the recipe below. The chicken turns out okay, but the bottom of the pan ends up looking like the aftermath of a forest fire, completely blackened and charred. The onions and potatoes are burned as well. How can I prevent this from happening? The link to the recipe video and the recipe steps are below.

Recipe video

Recipe:

Ingredients:

1 whole chicken
2 medium sized onions
1 lemon
1 garlic bulb
2 tablespoons of butter
1 tablespoon of chopped thyme, and 6 thyme sprigs
1.5 teaspoons of salt
1 teaspoon of pepper
1 tablespoon of oil
6 baby potatoes

Steps:

1. Pat the chicken dry with paper towels and place it in a large roasting pan.

2. In a small bowl, stir together melted butter, oil, chopped thyme, salt, and pepper.

3. Gently loosen the skin on the chicken breast and thighs and rub the butter mixture under the skin and all over the outside of the chicken. Squeeze the lemon over the chicken.

4. Place the lemon rinds in the cavity along with the halved garlic and 3 thyme sprigs.

5. Using kitchen twine, tie the legs together. Place the onion slices and the remaining 3 sprigs of thyme around the chicken in the bottom of the pan.

6. Place the chicken into a 450°F (232°C) oven and reduce the temperature to 425°F (218°C). Bake until golden brown and a thermometer inserted in the thickest part registers 165°F (74°C), about 1 hour 15 minutes to 1 hour 30 minutes, basting with juices every 30 minutes. Let rest at room temperature for 15 minutes before carving. Serve with roasted onion and pan drippings.