Cut squash in half; discard seeds. Place squash cut side down in a greased 15x10x1-in. baking pan; add 1/2 in. of hot water. Bake, uncovered, at 350° for 35-40 minutes. Drain water from pan; turn squash cut side up. Bake 10 minutes longer or until squash is tender; cool slightly. Carefully scoop out squash; place in a large bowl and mash. Set aside., In a large 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 the squash, onion mixture, feta cheese, salt, pepper and cayenne if desired. , Transfer to a greased 11x7-in. baking dish. Sprinkle with sunflower kernels. Cover and bake at 375° for 25 minutes. Uncover; bake 25-30 minutes longer or until a thermometer reads 160°.