Day before: dissolve the salt in the cold water in a large container. Submerge the bird, refrigerate 12 to 24 hours.
Day of: lift the bird out, pat very dry with paper towel. If you have time, leave uncovered on a rack in the fridge for another 12 hours to dry the skin further (the morning of works at a pinch).
Preheat oven to 220°C fan-forced. Mash the softened butter with the crushed garlic, thyme leaves and a pinch of pepper.
Gently slide your fingers between the breast meat and the skin to loosen. Push the herb butter under the skin, spread it across the breast.
Stuff the cavity with the lemon halves, thyme sprigs, whole garlic cloves and bay leaf. Truss the legs with kitchen string if you have it; tuck the wing tips under.
Place breast-side-up in a wide cast-iron pan. Season the skin with cracked pepper. Roast for 50 minutes for a 1.6kg bird (add 5 minutes per 200g over 1.6kg).
Test for doneness: juices should run clear from a thigh pierced at the joint, or a probe in the thickest part of the thigh should read 75°C.
Lift the bird onto a warm plate. Tent loosely with foil. Rest 15 minutes (non-negotiable).
Make gravy: pour off most of the fat from the pan. Place the pan on medium heat. Whisk in the flour to the dark fond. Add stock and wine, whisk hard, simmer 5 minutes until thickened. Strain into a warm jug.
Carve the rested bird. Sprinkle with flaky salt. Serve with the gravy poured over.