Reduce wine and port by three-quarters in a pan on high heat.
Add garlic, bay, thyme, and lemon zest to wine mixture and cool.
Rub marinade into chicken and under skin, then refrigerate overnight.
Rest chicken at room temperature for one hour before cooking.
Preheat oven to 150°C.
Bake chicken covered in foil for one hour and reserve juices.
Whisk egg yolks and mustard until pale, then slowly emulsify with rapeseed oil.
Mix in vinegar, seasonings, lemon juice, and celeriac, then chill.
Preheat barbecue to 150°C and oil the grill bars.
Grill chicken skin-side down for seven minutes with lid closed.
Flips chicken, baste with juices, and grill for five more minutes.
Rest chicken for ten minutes, joint it, and serve with remoulade.