Heat butter in a large frying pan; cook onions until softened and golden.
Add rabbit and carrots and stir. Work in flour, salt and black pepper; gradually add milk until mixture is thick and creamy. Transfer into a casserole dish and cook covered in the oven for about 45 minutes.
For the pastry: Combine the self raising flour and suet in a mixing bowl. Mix with enough water to make into a dough. Work together into a ball with fingers until it leaves the bowl clean.
Transfer pastry to a floured board; roll and shape out to same dimensions of your casserole dish. Secure the pastry on top of the casserole dish; bake at 200 C / Gas 6 for about 30 minutes.