Preheat the oven to 190C/375F/Gas 5.
Heat a large knob of butter in the base of a lidded flameproof casserole large enough to fit all the rabbit pieces in. Initially, you need to fry the rabbit in two batches. Don't cram it all in at once, or you won't get the desired browning.
Season the rabbit pieces with salt. You may dust them with flour - this is not necessary, but will produce a thicker sauce at the end of the cooking process.
When the butter is starting to turn golden-brown, add half of the rabbit pieces. Don't stir them, but leave them to brown. After two minutes, turn the pieces over and brown the other side. Remove the pieces and repeat the process with the remaining rabbit. You may need to add another knob of butter. Remove the second batch of rabbit and set aside with the first.
Add two tablespoons of water to stop the browned bits at the bottom of the pan from burning. Add the vegetables and pancetta pieces, adding a little splash of olive oil. Fry for a few minutes. Add the bay leaves, nutmeg and thyme and fry over a low heat for about 20 minutes, until the vegetables are softened and golden-brown.
Nestle the rabbit pieces among the vegetables and pour in as much cider as is needed to cover them.
Add the peas and reserved liquid from one can, then cover with the lid. Transfer the casserole to the oven and cook for one hour.Remove the casserole from the oven and stir in the Dijon mustard and red wine vinegar. Add a splash more cider if the rabbit has become uncovered. Return to the oven for another hour. Remove from the oven and remove the lid. Place over a medium heat on the hob.
Add the butter and lettuce and stir it to combine. As soon as the lettuce has wilted, but the stalk is still obviously crisp, the dish is ready to be served.
Serve at the table in the casserole, or transfer to a nice serving dish, and scatter the mint over the top. Serve with boiled potatoes or a creamy mashed potato.