If using, cook the red pepper in the broiler, oven, or over the flame of a gas stove until slightly charred. Remove from the heat and put in a brown paper bag or cover with paper towels. Once cool, remove the skin. Slice, de-seed, and set aside.
Meanwhile, cook the pasta in a large pot of salted water until al dente. Reserve 1 cup of pasta water before draining.
To make the sauce, combine the cilantro, parsley, scallions, garlic, and ¼ cup olive oil in a food processor. Blend until a paste forms. Add the lemon juice and Parmesan and season with salt and pepper. Taste and add more garlic, lemon juice, olive oil, Parmesan, salt, and/or pepper as needed. Add a dash of the pasta water and blend until smooth. Toss with the pasta and roasted red pepper and add more pasta water as needed. Serve with additional Parmesan for grating on top.
Recipe by Katherine Martinelli at http://www.katherinemartinelli.com/blog/2011/no-cook-herb-pasta-sauce/