This is not at all the recipe I remember. My mother had an old tattered Lea and Perrins cookbook from the 50's or 60's and indeed it was her secret ingredient. I remember two recipes from that book. The baked onions and cheese and the Baked Yams. This is the way I remember it. It may be that my mother adapted this recipe for her tastes (mine too); no frozen onion (why would any one use frozen) and Cheddar Cheese (American? yuk). Don't know the quantities, I just do it.
The rest is basically the same with these exceptions:
Slice large sweet onion and par boil them...Saute in butter.. Layer with shredded cheddar cheese...Top with buttered soft bread crumbs... and my brother's requirement: bake till bread crumbs and cheese are almost burnt.