Bring a large pot of lightly salted water to a boil. Add pasta and cook for 8 to 10 minutes or until al dente; drain.
Meanwhile, melt butter in a large frypan over medium-low heat. Stir in onion and garlic and cook 5 minutes, or until onion is translucent. Add clams, stirring frequently for 3 minutes or until clams are lightly browned. Stir in parsley, olive oil and reserved clam juice mixed with enough water to equal 2 cups liquid.
Increase heat and bring to a low boil. Reduce to a simmer, cover and cook 15 minutes, stirring occasionally. Serve over pasta with freshly grated Parmesan cheese, if desired.