Heat oven to 220C/200C fan/gas 7. Put the lemon halves in the chicken cavity. Stir all the ingredients for the curry butter together and season with salt and lots of pepper. Use your fingers to stuff the curry butter under the skin and smear it all over the meat.
Place the chicken in a flameproof roasting tin, on a trivet, if you have one. Roast for 20 mins, then turn the oven down to 180C/160C/gas 4. Continue to roast for 40 mins, or until the chicken is cooked through. Remove the chicken from the tin and leave to rest while you make the sauce.
If your roasting tin is flameproof, place it directly on the heat, if not, scrape all the buttery goodness from the tin into a saucepan and set over a low heat to make the sauce. Gently sweat the garlic and ginger in the curry butter. Scatter in the cardamom, cloves and fennel seeds and cook for 2 mins, then add the ground spices and cook for another 2 mins. Pour in the passata and gently reduce by half before adding the cream and reducing by a third. To finish the sauce, pour in the resting juices, season and add a squeeze of the roasted lemon from the cavity of the chicken. Carve the chicken and serve with the sauce.