Dredge rabbit pieces in flour until evenly coated.
Heat vegetable oil in a heavy-based casserole over medium heat.
Brown rabbit pieces in batches for 6-8 minutes and set aside.
Fry leek, onion, celery, peppercorns, halved garlic heads, and herbs for 5 minutes.
Add wine, boil, and reduce by half.
Add stock and rabbit, then simmer uncovered for one hour until meat is tender.
Transfer cooked rabbit to a warm plate and cover.
Boil the remaining liquid until the sauce thickens and reduces.
Stir in double cream and simmer for 10-12 minutes.
Strain sauce through a sieve, return rabbit to the pan, and warm through.
Crush remaining garlic cloves into a paste and stir into the sauce.
Fry mushrooms in butter until golden and add to the casserole.
Spoon casserole onto plates and garnish with chives.