Heat large skillet over medium heat. Add oil, onion, and garlic. Cook 2 min. Add tomato paste, cook 1 min. Add tomatoes and puree, sugar, salt and red pepper flakes. Break up tomatoes with wooden spoon. Cook, uncovered over medium heat for 12 min. Stir occasionally.
Cook pasta as directed, drain, reserving 1/2 cup cooking water. Tear basil into bite-size pieces, stir into sauce.
Toss pasta with sauce, adding 1/4 cup reserved liquid. Spoon pasta into bowls. Top each with 1 Tbsp. ricotta cheese.