Cook onion, poblanos, and 1/2 teaspoon salt in oil in a heavy medium pot over medium heat, covered, stirring occasionally, until onion is golden, 8 to 10 minutes
Add red peppers and cook, stirring occasionally, until onion is tender, 2 to 3 minutes
Meanwhile, cook ziti in a pasta pot of boiling salted water (2 tablespoons salt for 6 quarts water) until al dente