Cut top off each tomato. Scoop out pulp, leaving shells intact. Chop and reserve 3/4 cup pulp. Discard tops and remaining pulp, or save pulp for another use. Invert tomato shells on paper towels; let stand 30 minutes.
Place a large nonstick skillet coated with cooking spray over medium heat until hot. Add onion and sauté 5 minutes or until tender. Add tomato pulp, spinach, couscous, rosemary, salt, and pepper; cook 2 minutes or until thoroughly heated. Remove from heat, and stir in feta cheese.
Spoon about 3/4 cup filling into each tomato shell. Place tomatoes into an 8-inch square baking dish coated with cooking spray. Bake at 350° for 25 minutes or until thoroughly heated.