How to Make It
Heat oven to 450° F. In a 9-by- 13-inch baking dish, mix together the sugar, vinegar, oil, 2 tablespoons water, and ¼ teaspoon each salt and pepper. Add the squash, onion, and thyme sprigs and toss to coat. Cover with foil and bake, tossing once, until beginning to soften, 20 to 25 minutes. Uncover and continue to bake, tossing once, until tender and glazed, 15 to 20 minutes more. Remove thyme springs and gently mash with a potato masher.
Place the baguette slices on a baking sheet. Dividing evenly, top with the squash mixture and Gruyère. Bake until melted, 5 to 7 minutes. Sprinkle with the thyme leaves.