Combine the mustard powder, garlic granules, 1 tsp salt, 1 tsp pepper, smoked paprika, and 2 tbsp chopped fresh rosemary into a small bowl. Stir to combine.
Place the chicken in a large container suitable for marinating it in the fridge overnight. Drizzle the chicken with olive oil and rub it in thoroughly with clean hands. Then add 1/2 cup of the orange juice and rub that in throughly as well. Stuff the chicken with the flesh of the orange that's been squeezed.
Sprinkle the seasoning mix over the chicken evenly across all the skin. Pat it in gently--rubbing is ok too but will make the dry seasoning get clumpy, which prevents it from having a more even flavor/color once cooked. There should be an even coating of spice covering all corners of the chicken. Cover the chicken with a lid or tin foil and marinate overnight.
When ready to roast, let the chicken sit on the counter for 30 minutes to warm up slightly. Preheat the oven to 375º F. Place the chicken on a baking sheet lined with parchment paper.
Slice and prep all your vegetables. Coat them evenly in a drizzle of olive oil and salt and fresh pepper. Add the sprigs of fresh rosemary and stir to combine. Assort half of your vegetables around the chicken on that baking sheet, and then lay the rest on a second baking sheet lined with parchment paper. The goal is to have room around each vegetable so that air will circulate. This makes them get crisp and golden rather than soggy.
Bake the chicken for 30 minutes, and then remove it from the oven briefly to baste it. Using a basting brush, very gently dampen the chicken with any fat that has run-off. Pour 1/4 cup of the reserved orange juice onto the chicken. Return to the oven and bake for 30 minutes more, and then remove from the oven to baste again and pour the final 1/4 cup of orange juice over the chicken.
Return to the oven for 60 minutes more, or until an internal-read thermometer reaches 165º F. You can baste a third time during this timer.
When the chicken has 30 minutes left on its final 60-minute timer, add the second sheet pan of vegetables to the oven. They should be golden and cooked through when the chicken finishes its roasting.
Allow the chicken to rest for 10 minutes before carving. Slice, and then serve with the roasted vegetables and any reserved juices. Enjoy!