Saute the onion and mushrooms in butter until the mushrooms have release their liquid and it has evaporated. Mix in the garlic an saute 1 more minute.
Transfer the mushrooms to a food processor with the walnuts and 1 cup arugula. Pulse to break up into pea sized pieces. Add the olive oil and lemon juice and pulse until you reach your desired consistency. Stir in salt, pepper, and parmesan. Taste test and adjust seasonings to taste.
Meanwhile, cook pasta according to package directions. Reserve ½ cup pasta cooking liquid before draining.
Return pasta to the pot and add the pesto. Stir vigorously adding pasta cooking liquid 1 TBS at a time until the pasta is coated. I used 5 TBS pasta cooking liquid. Fold in the remaining 1 cup arugula.
Serve garnished with additional parmesan to taste.