1. Let frozen cod stand at room temperature 15 minutes to thaw slightly.
2. In 10-inch skillet over high heat, heat water, 1 tablespoon vinegar, and 1 teaspoon salt to boiling.
3. With serrated knife held in slanting position, cut block of frozen fish crosswise into 1/2-inch-thick slices. Add cod to skillet; over high heat, heat to boiling. Reduce heat to low; cover and simmer 5 minutes or until fish flakes easily when tested with a fork. With slotted pancake turner, remove fish to platter; keep warm.
4. In 1-quart saucepan over high heat, heat olive oil, tarragon, chives, pepper, 2 tablespoons vinegar, and 1/4 teaspoon salt to boiling. Pour sauce over fish. Garnish with lemon slices.