In 2-quart casserole, cook onion and olive oil on HIGH 3-4 minutes, until tender, stirring halfway through cooking. Stir in rice, coating each grain. Stir stock and salt into rice. Cook, covered, on HIGH 5-8 minutes, until boiling. Stir. Cook, covered, on MEDIUM (50 % power) 13-15 minutes, until liquid is absorbed and rice is tender. Drain spinach well, squeezing to remove all liquid. Stir spinach, Parmesan, and pepper into rice. Let stand, covered, 5 minutes. Remove seeds from tomato and cut into 1/2" chunks; stir into rice.