Preheat the oven to gas 6, 200°C, fan 180°C. Place the poussins in a large roasting tin and dot with the butter. Season well. Zest the two lemons and reserve, then cut lemons in half and place in the roasting tin with the onions, 5 garlic cloves, the stalks from the parsley and the carrots.
Pour over 250ml (8fl oz) water and roast for 1 hour until completely cooked and the juices run clear when a skewer is inserted. Finely chop the remaining garlic clove and parsley and sprinkle over the birds with the reserved lemon zest 5 minutes before the end of cooking.
When cooked, remove the birds and keep them warm in the oven. Put the roasting tin on the hob, add the stock cube and a little extra water if needed, and bring to the boil. Mix the cornflour with 1 tbsp cold water and stir into the pan juices. Boil for 1 minute until thickened.
Put a bird on each plate and serve with the warm gravy, roasted lemon, green beans and roasted new potatoes.
For top tips on protecting you and your family when preparing raw meat and poultry, visit Food Safety in the Home and view the full Food safety at home video.