Remove the front legs by pulling each leg away from the neck and breast, then cut straight through the seam to remove the top part of the shoulder.
Remove the neck and shoulder by cutting between the 4th and 5th ribs. Debone both parts of the shoulder, keeping the bones and meat separate.
Locate where the ribs finish and meet the loin, then chop through the vertebra at that point. Separate the loin from the hind legs by finding the edge of the hip bone and cutting through.
Trim the racks, clean the bones, and set the meat aside. Debone the loin as a saddle, keeping it fully intact with no holes for stuffing. Remove the hip bone and femur from the hind legs. Reserve all bones and trimmings.
To begin the rabbit stock, roast the reserved bones until golden brown.
In a heavy-based pot, heat a splash of grapeseed oil and sweat the diced carrot, celery, onion, and 3 garlic cloves until translucent. Pour in the white wine and reduce by half.
Add the roasted bones and bouquet garni to the pot, then cover with approximately 2 liters of water. Bring to a very gentle simmer and cook for at least 3 hours.
Prepare the stuffing (farce) for the loin: place all trimmings and boneless shoulder meat into a food processor with a few pinches of salt and pepper. Pulse for a few seconds until a paste begins to form.
Add the remaining 3 garlic cloves, wine, parsley leaves, and thyme leaves (stems removed). Blend until the farce is completely smooth. Adjust seasoning with salt if needed, then fold through the diced pork back fat.
Transfer the farce into a piping bag. Lay the boneless loin flat and pipe the farce along the center between the two sides of the loin. Roll up tightly in several layers of cling film, securing the ends tightly.
Submerge the wrapped loin into the simmering stock and poach for about 45 minutes, or until the internal temperature reaches 60°C (140°F). Remove from the stock and set aside. (Any leftover farce can be rolled in cling film, tied like a sausage, and poached alongside to serve as rabbit mortadella.)
Truss the semi-boneless hind legs with kitchen twine into an even shape. Season with salt and brown in a skillet with butter. Transfer the legs to a baking dish, pour over the simmering stock and its aromatics, and bake uncovered at 180°C (350°F) for 40 minutes.
Remove the dish from the oven, transfer the legs to a warm plate to rest, and strain the cooking liquid into a clean saucepan.
Whisk the Dijon mustard, cream, and lemon zest into the strained cooking liquid. Simmer gently until the sauce reduces and coats the back of a spoon. Season with salt and pepper to taste, and keep warm over low heat.
While the sauce reduces, peel the baby potatoes and boil gently until fork-tender. Drain and let steam dry.
Heat olive oil in a skillet over high heat. Unwrap the poached loin and sear briefly until golden on all sides, then remove to rest. Flash-fry the sage leaves until crisp.
Toss the cooked potatoes in the hot oil until well coated and transfer to a serving bowl.
Slice the loin and legs into portions, arrange on warm plates, nap generously with the mustard sauce, and serve accompanied by the crispy sage, potatoes, and baby cornichons.
