Fill large bowl with ice and cold water. Cook brussels sprouts in large pot of generously salted boiling water 2 minutes. Add cauliflower to same pot; cook until vegetables are crisp-tender, about 3 minutes longer. Drain. Transfer vegetables to bowl of ice water to cool. Drain well.
Combine cream, shallots, and sage in large saucepan. Bring to boil. Reduce heat; simmer until mixture is reduced to 21/2 cups, about 10 minutes. Season with salt. Remove from heat. Cool slightly.
Heat oil in large nonstick skillet over medium heat. Add breadcrumbs; stir until beginning to brown, about 2 minutes. Transfer to bowl; cool. Stir in pine nuts and parsley. Season with salt and pepper.
Butter 13x9x2-inch glass baking dish; arrange half of vegetables in dish. Sprinkle with salt and pepper, then 1 1/2 cups Parmesan. Arrange remaining vegetables evenly over, then sprinkle with remaining 1 1/2 cups Parmesan. Pour cream mixture evenly over.
Preheat oven to 375°F. Cover gratin with foil. Bake covered 40 minutes. Uncover; sprinkle breadcrumb topping over and bake uncovered 15 minutes longer.
Adapted from Bon Appétit, November 2008, by Lora Zarubin