Dissolve bouilon cube in hot water, and half and half in a saucepan, melt butter and add flour. Whisk and cook until mixture is frothy and raw flour taste is gone.
While stirring, add the bouillon mixture. Stir constantly with a whisk until the sauce thickens and begins to bubble. Add Swiss cheese and stir until smooth (add water and heat if sauce needs to be thinned)
Assemble hot browns by placing turkey and onion on each bread slice. Top with sauce and crumbled bacon. Heat at 350 degrees for 10 minutes.
Garnish with parsley.