Step 1 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.
Step 2 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.
Step 3Pour 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.