Preheat oven to 375˚F.

Fill a large bowl with ice and cold water. In a large pot of salted boiling water, cook the Brussels sprouts for 2 minutes. Add the cauliflower and cook until all the vegetables are crisp-tender, about 2 minutes. Drain and transfer immediately to the bowl of ice water, submerging them to stop the cooking. Drain well.

In a large saucepan, stir together the cream, shallots, and sage, and bring to a boil. Reduce heat to a simmer, and reduce the mixture to about 2 ½ cups, about 10 minutes. Sprinkle with salt and remove from heat. Set aside.

In a large skillet over medium heat, warm the oil. Add the breadcrumbs and cook, stirring, until they begin to brown, about 2 minutes. Transfer to a small bowl and allow to cool. Stir in the pine nuts and parsley, then sprinkle with salt and pepper.

Lightly butter a 9x13-inch baking dish. Sprinkle with salt and pepper, then with 1 cup of the Parmesan. Layer the Brussels sprouts and cauliflower over the cheese, and sprinkle with the rest of the Parmesan. Pour the cream mixture over the vegetables, and cover the dish with aluminum foil.

Bake covered for 40 minutes. Uncover, then top evenly with the breadcrumb topping, and bake uncovered for 15 more minutes. Enjoy warm.