Sauté onion in butter in a saucepan until tender. Add milk; cook over low heat 5 minutes. Add 1 cup cheese; cook, stirring constantly, until cheese melts. Cool.
Stir in egg yolk, salt, pepper, and paprika. Spoon 1/4 cup of sauce into a 2-quart baking dish. Place cauliflower in sauce. Spoon remaining sauce over cauliflower. Top with remaining cheese and breadcrumbs.
Bake at 350° for 15 minutes or until lightly browned.