Preheat your oven to 425º, making sure the rack is in the center position. This temperature ensures the chicken cooks evenly and the skin becomes crispy.
Cut the lemon in half widthwise and squeeze the juice into a small bowl, removing any seeds as you go. Set the squeezed halves of the lemon aside for stuffing the chicken later.
In a large bowl, combine the yellow onions and baby potatoes. Pour in the lemon juice you just squeezed, along with the whole-grain Dijon mustard, extra virgin olive oil, finely chopped rosemary, thyme, and 1/2 teaspoon each of kosher salt and pepper. Toss everything together so the vegetables are evenly coated.
Spread the seasoned onions and potatoes in an even layer on a sheet pan and set aside. This step allows for even roasting and delicious caramelization.
Next, check the roasting chicken for any giblets inside the cavity; remove and discard them. Use a sharp knife to trim off any excess fat from the chicken, especially around the cavity.
Pat the skin of the chicken dry with paper towels. This crucial step helps achieve that desirable crispy skin. Liberally season the chicken inside and out with kosher salt and pepper.
Now stuff the cavity of the chicken with the reserved squeezed lemon halves, the sprigs of rosemary and thyme, the bay leaf, and the garlic cloves. This will infuse wonderful flavors into the meat.
Rub unsalted butter all over the chicken skin. This not only adds flavor but also helps the skin to become beautifully golden brown. Sprinkle with another 1/2 teaspoon of salt and pepper.
Place the chicken, breast side up, on top of the bed of potatoes and onions. Use kitchen twine to tie the legs together tightly and tuck the wings under the chicken's body. This helps the chicken cook evenly and retain its shape.
Roast the chicken and potatoes in the preheated oven until a thermometer inserted into the thickest part of the thigh reads at least 145º, and the juices run clear when pierced. If the potatoes start to over-brown, carefully remove them with a spatula and set them aside until it's time to serve.
Once done, allow the chicken to rest for 10 minutes before slicing into pieces. This resting time allows the juices to redistribute, ensuring every bite is juicy and tender. Serve the chicken alongside the roasted onions and potatoes for a complete meal.
