Preheat oven to 400°F. Melt butter in medium saucepot over medium heat. Add onion and salt and cook 3 minutes or until softened. Stir in flour and garlic and cook 2 minutes, stirring constantly.
Gradually whisk in broth until smooth. Bring to a boil over medium heat, stirring occasionally. Reduce heat to medium-low and simmer 3 minutes or until thickened. Stir in 1 cup cheese until melted and smooth. Stir broccoli into sauce and cook 1 minute.
Turn broccoli mixture into 1-1/2 quart baking dish and top with remaining cheese. Cover and bake 10 minutes. Remove cover and bake 5 minutes or until bubbling and cheese is melted.