Coat the insert of a 5- to 7-quart slow cooker with nonstick cooking spray or line it with a slow-cooker liner according to the manufacturer's directions.
Melt the butter in a saucepan over medium-high heat. Add the onion and sauté until the onion is softened, about 2 minutes. Stir in the flour, and cook for 2 to 3 minutes, whisking constantly. Gradually add the broth and milk and bring the sauce to a boil, whisking constantly.
Add the Tabasco and remove the sauce from the heat. Add 3 cups of the cheese to the sauce and stir to melt. Spread half the macaroni in the slow-cooker insert and pour half the sauce over the macaroni.
Sprinkle 1 cup of the cheese over the layer. Repeat with the remaining macaroni and sauce and 1 cup of cheese. Cover and cook on low for 4 to 5 hours, until the mac and cheese is bubbling and cooked through.
Sprinkle a little more cheese and the crackers over the top of the mac and cheese. Remove the cover and cook for an additional 30 minutes.
Allow the mac and cheese to rest about 10 minutes.
Serve in the cooker set on warm.