Place the potatoes in a saucepan with enough cold water to cover. Bring to a boil, cover, and reduce heat to medium.
Cook 10 minutes or until just tender. Drain and place in a bowl with the clam sauce. Set aside.
Meanwhile, cook the fettuccine according to the package directions. Drain.
Return the pasta pot to medium heat and heat the oil and butter. Add the garlic and parsley and cook until fragrant, about 1 minute.
Pour in the potato mixture and bring to a simmer.
Add the cooked fettuccine and lemon juice and cook until heated through, 1 to 2 minutes.