Peel the onion then cut them into small slices. Cut lemon into slices, too.
Rinse the chicken and dry thoroughly with paper towels. The drier the chicken, the crispier the skin you will achieve.
Put the thyme, lemon slices, onion, 1 teaspoon of salt and pepper inside the chicken and rub it well.
Tie the legs and put the wings back so they are stuck under the chicken. By doing this, everything will be cooked evenly.
Rub olive oil and the remaining salt around the chicken skin. A uniform layer of salt will give the skin a great crispy taste.
Put the chicken on a baking tray to roast for 50-60 minutes.
In the meantime, and add melted butter and mustard in a bowl. Once the chicken is cooked, remove from the oven. Stir the mustard butter mixture and brush the chicken skin with this mixture.
Let rest for 15 minutes. Slice and enjoy with a low-carb salad or side dish.