Cut squash in half; discard seeds. Place squash cut side down in a greased 15-in. x 10-in. x 1-in. baking pan. Bake at 375° for 35-40 minutes or until tender; cool slightly. Carefully scoop out squash; place in a bowl and mash.
In a skillet, saute onion and garlic in butter until tender. Add peppers; saute until crisp-tender. In a large bowl, whisk eggs and yogurt until blended. Stir in squash, onion mixture, feta cheese, salt, pepper and cayenne if desired. Transfer to a greased 11-in. x 7-in. x 2-in. baking dish. Sprinkle with sunflower kernels.
Cover and bake at 375° for 25 minutes. Uncover; bake 25-30 minutes longer or until a knife inserted near the center comes out clean.