Reserve about one-third of the chopped tomatoes. Combine the remaining tomatoes, parsley, mint, basil, capers, garlic, red pepper flakes, sugar, salt, and black pepper in a food processor fitted with the metal blade; process until a smooth paste is formed. With the motor running, add the oil in a thin, steady stream. Transfer the mixture to a large serving bowl.

Meanwhile, cook the fettuccine in boiling salted water according to package directions until al dente. Drain well and add to the bowl with the tomato mixture. Add the reserved tomatoes and toss like a salad until well combined. Serve immediately, garnished with the optional walnuts and parsley, if using.