Mix together the buttermilk, salt, pepper and thyme leaves.
Pour over the chicken, cover and refrigerate for at least 6 hours but ideally overnight.
Preheat the oven to 180°C/350°F.
Remove the chicken from the marinade then blot off any excess marinade with paper towels.
Place the chicken in a roasting dish then drizzle with olive oil and season generously with salt and pepper.
Add thyme, fresh sage and garlic cloves. Place in the oven and allow to roast for 20-30 minutes (roasting time will differ if using smaller or larger pieces of chicken) until the chicken is golden brown, crisp and cooked through.
Remove from the oven and allow to rest for 5 minutes then serve.