Lay each squash on side. Cut about one-third of each lengthwise in wedge; peel wedge, dice and reserve.
Scoop out seeds from remaining squash; cut thin slice off each to level with cut side up. Place in microwaveable dish; microwave on High for 12 to 15 minutes or until tender.
Meanwhile, in nonstick skillet, melt butter over medium heat; cook reserved diced squash, onion and celery, stirring occasionally, until softened, 5 minutes. Add mushrooms and garlic; cook for 2 minutes. Stir in tomato paste; cook for 1 minute.
In bowl, combine onion mixture, bread crumbs, Cheddar cheese, sunflower seeds, salt and pepper. Divide between squash, mounding if necessary; sprinkle with Parmesan cheese.
Bake in oven for 7 to 10 minutes or until golden and heated through.