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