Begin by slicing the brioche loaf.
Next, thinly slice the sweet and mild onion.
Spread mayonnaise on one side of each slice of brioche.
On one slice of brioche, layer the sliced onions over the mayo.
Sprinkle salt over the onions.
Add chopped parsley on top of the onions.
Place another slice of brioche on top, mayo side down.
Gently press down on the sandwich and cut it in half if you prefer.
