3 Tbs. unsalted butter
3 Tbs. good olive oil
3 cups (10 1/2 oz./330 g) halved and thinly sliced yellow onions (2 large)
1 Tbs. minced garlic (3 cloves)
2 lb. (1 kg) butternut squash, peeled, halved, seeded and sliced crosswise 1/8 inch (3 mm) thick
1/2 tsp. ground nutmeg
Kosher salt and freshly ground pepper
1/2 cup (4 fl. oz./125 ml) heavy cream
2 cups (4 oz./125 g) coarse fresh bread crumbs from a country loaf
1/2 cup (2 oz./60 g) grated Gruyère cheese
Preheat the oven to 350 degrees. Use 1 Tbs. of the butter to grease an 8 1/2-by-11 1/2-inch (21.5-by-29-cm) oval baking dish or baking dish of similar size.
In a large (12-inch/30-cm) pot or Dutch oven, heat the remaining 2 Tbs. butter and 1 Tbs. of the olive oil over medium heat. Add the onions and garlic and cook, stirring occasionally, for 6 to 8 minutes, until tender. Stir in the squash, nutmeg, 2 tsp. salt and 3/4 tsp. pepper, cover and cook for 10 minutes, stirring occasionally.
Spoon the squash mixture into the prepared baking dish and smooth the top, making sure all of the squash slices are laid flat. Pour the cream over the mixture. In a medium bowl, combine the bread crumbs and the remaining 2 Tbs. olive oil and mix in the Gruyère. Sprinkle evenly over the squash. Bake for 35 to 40 minutes, until the top is browned and the squash is very tender when tested in the center with a small knife. Sprinkle with salt and serve hot. Serves 6.