Onions lose their pungency and gain a sweetness when cooked slowly.
Preheat oven to 400 degrees. Arrange onion halves in a large, nonreactive baking dish; season with salt. Add 3/4 cup water to dish. Cover with foil. Bake onions until tender, 40 to 45 minutes.
Meanwhile, melt butter in a large skillet over medium heat. Add chopped onion. Cook, stirring, until softened, about 4 minutes. Stir in breadcrumbs. Cook, stirring occasionally, until breadcrumbs are golden, about 3 minutes. Add lemon juice; season with salt.
Top onions with breadcrumb mixture, dividing evenly. Return to oven, and bake until topping is crisp and golden brown, 20 to 25 minutes more.