Preheat oven to 450°. Trim stalks from fennel bulbs. Cut bulbs lengthwise into 1/2-inch-thick wedges; chop 2 Tbsp. fronds. Toss wedges with yellow squash, onion, olive oil, pepper, and salt. Place in a lightly greased large roasting pan. Bake 25 minutes. Stir once, and bake 15 minutes or until fennel is tender and begins to caramelize. Sprinkle with chopped fronds.