Arrange cauliflower in a steamer basket over boiling water. Cover and steam 10 minutes or until crisp-tender. Place in a lightly greased 2-quart baking dish.
Melt butter in a heavy sauce-pan over low heat; whisk in flour, whisking until smooth. Cook, whisking constantly, 1 minute. Gradually whisk in milk; cook over medium heat, whisking constantly, until thickened and bubbly. Add cheese and next 4 ingredients, stirring until cheese melts. Stir in 1 1/2 cups crushed onions. Pour mixture over cauliflower.
Bake at 350° for 25 minutes. Sprinkle with remaining crushed onions, and bake 5 more minutes.