COOK spaghetti as directed on package, omitting salt. Meanwhile, heat 2 Tbsp. dressing in large skillet on medium-high heat. Add meat; cook 5 to 6 min. or until done, stirring occasionally. Remove from skillet; cover to keep warm.
ADD remaining dressing, onions and garlic to skillet; cook and stir 2 to 3 min. or until onions are crisp-tender. Add tomatoes; cook 1 min. Add broth and meat; stir. Bring to boil, stirring occasionally.
DRAIN spaghetti. Add to ingredients in skillet along with the spinach and 1/2 cup cheese; mix lightly. Top with remaining cheese.