Preheat oven to 450°F. Quarter squash lengthwise, then cut crosswise into 1-inch pieces. Combine the squash with sliced fennel, oil, thyme, salt and pepper in a large bowl. Spread the mixture evenly on a large, rimmed baking sheet. Roast for 10 minutes. Stir in garlic and roast until the vegetables are tender and the fennel is beginning to brown, about 5 minutes more. Stir in fennel fronds and serve.