Into a medium saucepan over medium heat, add the 3 tablespoons of butter. When the butter is melted, whisk in the flour until smooth; the mixture will be thick. Whisk in the milk, salt, onion powder, pepper, and nutmeg. Bring just to a boil, whisking continuously, then reduce the heat to low. Add the cheese and continue cooking, stirring frequently until the cheese is melted, about 3 minutes. Pour the cheese mixture evenly over the broccoli.