Preheat the oven to 200°C / 400°F / Gas mark 6.
Place the poussins in a large roasting tin and dot with the butter.
Season well with salt and pepper.
Zest the lemon and reserve, then cut lemon in half and arrange in the roasting tin with the onion, 1 garlic clove, the stalks from the parsley and the carrots.
Pour over 125ml of 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 a few minutes before the end of the cooking time.
When cooked, remove the birds and keep them warm in the oven.
Put the roasting tin on the hob, add the stock cube and bring to the boil adding a little water if needed. Mix the corn flour with 1 tablespoon of cold water and stir into the pan juices.
Boil for 1 minute until it thickens.
Serve with the gravy, roasted lemon, green beans and roasted new potatoes.