Combine the onion, vinegar, sugar, and 1 teaspoon pepper in a medium bowl; marinate in refrigerator 1 hour.
Drain the oysters in a colander over a bowl, reserving 1 cup oyster liquid. Combine reserved oyster liquid, juice, and Champagne in a large saucepan, and bring to a boil. Add oysters; reduce heat, and simmer 4 minutes or until the edges of oysters curl. Drain oysters in a colander over a bowl, reserving 1 cup cooking liquid.
Heat the oil in a large nonstick skillet over medium-high heat. Add garlic; sauté for 1 minute. Add 1 cup reserved cooking liquid, 6 teaspoons pepper, salt, and bay leaves. Bring to a boil, and cook for 30 seconds. Remove from heat; cool. Combine oysters and cooking liquid; marinate in refrigerator 30 minutes. Drain oysters; serve over onion mixture.