Sauté onion and garlic in oil in large skillet over medium heat until fragrant, about 1 minute. Crumble turkey sausage into skillet; sauté, breaking up pieces with wooden spoon, until slightly browned, about 7 minutes.
Stir in diced tomatoes, half-and-half, salt and pepper; simmer, uncovered, stirring occasionally, for 15 minutes or until slightly thickened.
Dissolve cornstarch in water in small bowl. Gently stir into sausage mixture. Add peas; cook until heated through.
Meanwhile, cook ziti in large pot of lightly salted boiling water until al dente, firm but tender. Drain well.
Transfer ziti to large serving bowl; top with sausage sauce. Makes 6 servings.
