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.