FILL Dutch oven with water; bring to boil. Meanwhile, cut small "x" in bottom of each tomato. Carefully add tomatoes to boiling water; cook 20 sec. Drain. Remove and discard peels and seeds from tomatoes. Finely chop tomatoes; set aside.
HEAT dressing in same pan on medium heat. Add onions; cook 5 min., stirring frequently. Add tomatoes; cook 20 min. or until thickened, stirring frequently.