Preheat the oven to 160C/325F (no fan) and prepare a roasting tray with a piece of lightly greased baking parchment paper on top of it.
Remove the bag of giblets from the bird's cavity (if there are any) and snip the ties if the capon is trussed. Quarter the onion and lemon, and place them inside the bird with the garlic cloves and thyme.
Rub the olive oil all over the bird and season generously with salt and cracked black pepper. Turn the bird upside down and place it onto the parchment paper on the roasting pan. Oil and season the underside, too.
Roast the capon for 3-4 hours, starting upside down for 1 hour.
After 1 hour, flip the bird the right way up, peel off the parchment paper, and drain off some of the juices into a jug. Return to the oven and roast for another hour.
Flip the capon upside down again, replacing the parchment paper under the breast. Drain off more of the meat juices. Roast for another hour.
Flip the bird the right way up and peel off the parchment paper. Take the internal temperature with a meat thermometer to see where you are. The goal is 74C/165F. The likelihood is you will need 30-45 minutes more in the oven, but be led by meat temperature, not time.
Roast the capon for a further 30-45 minutes,
Take the capon from the oven, and allow it to rest for 30 minutes. Place it on a carving board or serving platter, and cover it with aluminum foil. Then, drape several tea towels over the top and tuck them around it to keep the bird warm.
Carve and serve the capon chicken once it has rested.