When making your gameplan for big meals, remember most of this one can be made ahead of time! The squash and onion portion can be prepared 1 day ahead. Simply cool, cover, and refrigerate. When you're ready, reheat in a 350°F oven until hot and proceed with bread crumb topping.
Preheat oven to 350°F and butter a 9×13-inch oven to table baking pan.
In a large, heavy skillet over medium heat, melt the butter. When hot, add onions and cook, stirring constantly, until onions are transparent and light golden in color (about 4 to 5 minutes).
Add squash and cook, stirring 4 minutes more. Sprinkle sugar, 1/2 tsp each salt and pepper over vegetables, stir and cook until onions and squash start to lightly caramelize, 5 minutes more.
Spread squash and onions in prepared pan and pour chicken stock over it. Cover pan tightly with aluminum foil and place in oven. Bake 45 minutes, then remove foil.
Mix bread crumbs, Cheddar, rosemary, and thyme together in a mixing bowl and spread evenly over top of squash. Return to oven and bake until top is golden brown and crisp and squash is tender, about 30 minutes more.
Remove from oven and garnish center of dish with a bouquet of rosemary.