Cook the beans in a large saucepan of boiling water until crisp-tender, about 3 minutes. Drain and cool in a bowl of ice water. Strain and set aside in a shallow baking dish.
In a medium saucepan, combine the vinegars, mustard seeds, peppercorns, salt, sugar, fennel seeds, pepper flakes, garlic, dill and bay leaves over high heat. Bring to a boil, and cook for 1 minute to dissolve the sugar and salt.
Pour the vinegar solution over the beans to cover completely. Marinate, refrigerated for at least 24 hours. The beans will keep refrigerated for 3 to 4 weeks.