Bring a large pot of salted water to a boil. Add the pasta and cook until tender, but still firm to the bite, from 7 to 10 minutes. In a food processor puree until smooth the scallions, parsley, butter, tarragon, lemon zest, lemon juice and ricotta cheese (and optional heavy cream). Season to taste with salt and pepper. Drain the pasta, reserving 1/4 cup of the cooking water. Return pasta to pot, off heat, add the cheese mixture and reserved cooking water and toss thoroughly. Serve immediately.