Combine the lemon juice, sugar, Grand Marnier, triple sec, Cognac, and orange juice in a large nonreactive bowl and stir until the sugar is completely dissolved. Add the Champagne and stir to combine. Cover and refrigerate until chilled, about 1 hour.
Pour into a decorative bowl or pitcher. Garnish with orange or lemon slices and fresh strawberries, if desired. Serve in Champagne flutes, wineglasses, or punch cups.
Makes about 3 quarts, 12 to 16 servings