1. Cut squash in half lengthwise. Scoop out seeds; discard. Place squash halves, cut sides up, in a microwave-safe bowl. Cover with a damp paper towel. Microwave at HIGH 20 minutes or until tender. Let stand 10 minutes. Scrape inside of squash with a fork to remove spaghettilike strands to measure 4 cups.
2. Heat a large skillet over medium-high heat. Add spinach to pan; cook 2 minutes or until spinach wilts. Place squash and spinach on a clean dish towel; squeeze until barely moist. Coarsely chop squash mixture, and place in a large bowl. Add panko and next 4 ingredients (through baking powder), and toss well to combine. Place egg whites in a medium bowl; beat with a mixer at high speed until soft peaks form. Gently fold egg whites into squash mixture.
3. Fill a 1/4-cup dry measuring cup with squash mixture. Invert onto work surface; gently pat into a 3/4-inch-thick patty. Repeat procedure with remaining squash mixture, forming 10 patties total. Heat a large nonstick skillet over medium heat. Add 1 1/2 teaspoons oil to pan, and swirl to coat. Add 5 patties to pan; cook 3 minutes on each side or until browned. Remove patties from pan; keep warm. Repeat the procedure with remaining 1 1/2 teaspoons oil and squash patties.
4. Combine mayonnaise and remaining ingredients in a small bowl. Serve with fritters.