Heat water, wine, vinegar, 1/2 onion, lemon, sugar, salt, garlic, cloves, bay leaves, juniper berries, and peppercorns in a saucepan over medium heat until sugar is dissolved, about 5 minutes. Allow marinade to cool, then pour into a resealable plastic bag. Add chuck roast and toss to evenly coat, squeeze out excess air, and seal bag. Marinate in the refrigerator 2 to 3 days, turning roast twice a day.

Place 2 cups onions, carrots, and celery in a slow cooker. Remove roast from marinade; arrange on top of vegetables. Strain marinade; pour 2 1/2 cups over roast and reserve remaining marinade.

Cover slow cooker; cook on Low for 8 hours. Turn slow cooker off; transfer roast to a cutting board and cover with aluminum foil. Strain cooking liquid into a large bowl. Return vegetables to slow cooker; set aside.

Pour 3 cups cooking liquid (adding reserved marinade as needed to reach 3 cups liquid) into a saucepan; add gingersnaps. Heat over medium-high heat until gravy is thickened, about 10 minutes.

Slice roast; serve with vegetables and gravy.