Preparation Melt butter in heavy large skillet over medium heat. Add leeks, large onions, shallots and garlic and sauté until all are tender, about 20 minutes. Add baby onions and cook 10 minutes longer, stirring occasionally. Mix in 2 cups whipping cream. Boil until cream is thickened to sauce consistency, about 10 minutes. Transfer vegetable-cream mixture to 6-cup shallow baking dish. (Can be prepared 1 day ahead. Cover and refrigerate. Return mixture to room temperature before continuing.) Preheat oven to 425°F Sprinkle breadcrumbs over onion mixture. Bake until breadcrumbs are golden brown and onion mixture bubbles, about 20 minutes. Sprinkle with parsley.
Preparation Melt butter in heavy large skillet over medium heat. Add leeks, large onions, shallots and garlic and sauté until all are tender, about 20 minutes. Add baby onions and cook 10 minutes longer, stirring occasionally. Mix in 2 cups whipping cream. Boil until cream is thickened to sauce consistency, about 10 minutes. Transfer vegetable-cream mixture to 6-cup shallow baking dish. (Can be prepared 1 day ahead. Cover and refrigerate. Return mixture to room temperature before continuing.) Preheat oven to 425°F Sprinkle breadcrumbs over onion mixture. Bake until breadcrumbs are golden brown and onion mixture bubbles, about 20 minutes. Sprinkle with parsley.