Cut the tripe into 2 cm pieces. Cut 1 onion into 8 wedges. Place the tripe and onion wedges in a medium saucepan and cover with cold water. Stir in the salt. Cover with a lid and cook over low heat for 30 minutes, or until the tripe is tender. Drain, reserving 1 cup of the liquid and discarding the onion.
Meanwhile, finely chop the remaining onion. Melt the butter in a medium saucepan until hot and bubbling. Add the onion and cook until soft. Add the flour and stir constantly for 1 minute. Remove from the heat and add the combined milk and reserved 1 cup of cooking liquid in a slow, steady stream, whisking constantly to avoid the sauce becoming lumpy. Return the saucepan to the heat and cook, stirring constantly, on medium-high heat until the sauce comes to the boil and thickens.
Add the mustard, parsley and cayenne pepper and mix well to combine. Add the tripe, reduce the heat to low, cover the saucepan with a lid and cook, stirring occasionally, for 10 minutes, or until the tripe is heated through. Serve on warmed plates topped with crispy bacon and accompanied by slices of hot buttered toast.