On a plate pat dry chicken with a paper towel. Coat the chicken with 1½ tbsp sea salt. Store in the fridge for 24 hours (recommended) or overnight.
On the next day, take out the chicken from the fridge 30 minutes before cooking.
Preheat oven to 175°C/300°F. Put the shelf in the middle.
After 30 minutes, rub the whole chicken with 1½ tbsp room temperature butter. Sprinkle again with 1 tbsp sea salt. Stuff lemon, half of the garlic bulb, and 1 sprig of thyme inside the chicken.
Spread potatoes, carrots, and mushrooms on a roasting pan evenly. Add 1 rosemary sprigs, 1 thyme sprig, and half of the garlic bulb. Sprinkle with ½ tsp sea salt and a pinch of black pepper. Divide the ½ tbsp butter into 4 blobs and place it evenly on the roasting pan. Add the chicken on top of the vegetables.
Transfer the roasting pan to the oven and bake for 1 hour 15 minutes. And then turn the oven up to 220°C/350° for 15 minutes. You know the roast chicken is done by checking the temperature in the inside of the thigh is around 75°C/165°F or until juices run clear. Let the chicken rest at room temperature for at least 15 minutes.
Carve the chicken breast, thigh, and wings before serving.
Use the carrots, potatoes, and mushrooms as a tasty side dish.