Remove stems from spinach; wash leaves thoroughly in lukewarm water. Place in a large Dutch oven (do not add water). Cover and cook over high heat 3 to 5 minutes. Drain spinach well; chop and set aside.
Melt butter in a medium saucepan over low heat; add flour, stirring until smooth. Cook 1 minute, stirring constantly. Gradually add half-and-half; cook over medium heat, stirring constantly, until thickened and bubbly. Stir in salt and pepper. Add spinach and cheese; mix well.
Spoon mixture into a greased 2-quart casserole; sprinkle with breadcrumbs. Bake, uncovered, at 350° for 10 to 15 minutes.