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Roast the squash until the edges catch

Heat the oven to 400°F. Toss the squash, onion wedges and unpeeled garlic cloves with the oil, a teaspoon of the salt and the thyme, spread them over two large trays and roast for 40 minutes, turning once, until the squash is tender and browned at the corners.

— Two trays, not one. Squash crowded into a single layer that touches steams instead of roasting, and steamed squash is exactly what you were avoiding.