Add champagne to onion in skillet; cook until liquid has evaporated. Stir in rice. Cook uncovered over medium heat about 5 minutes, stirring frequently, until edges of rice kernels are translucent. In 4-cup glass measuring cup, mix chicken broth and clam juice; pour 1/2 cup mixture over rice. Cook uncovered, stirring occasionally, until liquid is absorbed. Repeat with remaining broth mixture, 1/2 cup at a time, until rice is tender and creamy.