1. Cook pasta in boiling water until almost al dente; drain through a sieve over a large bowl. Reserve 1/3 cup pasta cooking water.
2. Melt butter in a large skillet over medium heat. Add onion to pan; cook 4 minutes, stirring occasionally. Add garlic to pan; cook 1 minute, stirring constantly. Add wine to pan; cook 3 minutes or until the liquid evaporates, stirring occasionally. Add tomatoes; cook 2 minutes, stirring occasionally. Stir in reserved pasta cooking water and clam juice; bring to a boil. Add mussels; cover and cook 4 minutes or until mussels open. Discard any unopened shells. Add cooked pasta to the tomato mixture; cook 1 minute or until thoroughly heated, tossing well to combine. Sprinkle with parsley. Serve immediately.