Peel the onions and place them in a large saucepan. Add 1/2 teaspoon of the salt and enough water to cover.
Place the pan over high heat and bring the onions to a boil. Reduce the heat to low and cover the pan; cook for 20 to 25 minutes. Drain and set aside.
Melt the butter in a medium saucepan over medium-low heat; add the flour and the remaining 1/2 teaspoon salt. Cook the flour and butter slurry for about 2 minutes, stirring constantly.
Gradually add the milk; cook until the mixture thickens and comes to a boil, stirring constantly. Add the hot sauce, if using, and stir to blend. Taste and adjust seasonings.
Add the drained onions to the sauce; heat through and garnish with fresh chopped chives or parsley, if desired.