Cook pasta according to package directions. Reserve 1 cup of pasta liquid.
Heat olive oil over medium-heat in a large skillet. Add the chopped onion and saute for 2-3 minutes until soft and aromatic.
Add the garlic and cook for an additional minute, taking care not to brown it.
Add frozen clams, anchovy paste and balsamic vinegar along with reserved pasta liquid. Bring to a boil and then immediately reduce heat to simmer until liquid reduces by about half. Approximately 5 minutes.
Add pasta, salt, pepper and parsley. Mix well. Sprinkle with a little Parmesan cheese if desired and serve hot.