Sauté the diced onion for a few minutes over medium high heat, stirring occasionally. Close the lid for another few minutes, and continue to cook.
Once the onion is translucent, and just starting to brown, add the ground beef and break the pieces up into smaller bits. Stir often, and continue to cook until the liquid starts to dry (if not using Bisto, add a tablespoon of flour at this time).
Add enough water, just to almost cover the ingredients, and simmer. At this point, add salt, Beef Better than Bouillon (or one OXO cube), Bisto granules, Worcestershire Sauce, and some ground black pepper to taste.
Now, preheat oven to 375ºF (190ºC).
Continue to simmer the meat over medium heat for about 5-10 minutes, then taste; if needed, add more salt or Bisto. Remove from heat.
Ladle the beef and gravy into oven safe ramekins or bowls, to make individual Cottage Pies, or just one large dish. Spoon (or pipe) mashed potatoes onto each dish to cover completely. Add shredded Cheddar cheese on top, if desired.
Place dishes/ramekins onto a rimmed baking sheet and bake until bubbly and the potato is browned a little, approximately 25-30 minutes.