Place the rabbit pieces in a shallow pan.
Combine the onion, thyme, bay leaf, vinegar and water and pour the mixture over the rabbit.
Cover the pan tightly with plastic wrap or aluminum foil and allow the rabbit to marinate overnight in the refrigerator.
Remove the rabbit from the marinade and dry the pieces thoroughly.
Sprinkle with salt and pepper and roll in flour.
Heat the butter and brown the rabbit on all sides over high heat until golden.
Strain and add the marinade.
Lower the heat, cover the pan and simmer slowly for 30 minutes.
Add the drained prunes and brown sugar and simmer 15 minutes more.
Combine the beer and flour and stir until smooth.
Add the beer mixture to the rabbit, stirring constantly until the sauce is thickened.
Place the rabbit and prunes on a warm serving dish and pour the sauce over.