Speedy Spaghetti with Chicken and Fresh Tomato

uncooked spaghetti

extra-virgin olive oil

small onion, finely chopped

ripe plum tomatoes, seeded and cut in 1/4-inch dice

cooked boneless chicken, torn in pieces (3 cups)

salt

Freshly ground black pepper

chopped fresh basil

(4 to 5 oz.) log fresh goat cheese, crumbled