In a 10-inch skillet add the 1 tablespoon of butter. Heat over medium-low heat until melted. Add in the sliced onion and small pinch of salt and stir to coat in the butter. Let the onions cook, stirring occasionally until they are soft and lightly golden - or go longer for darker onions, if desired. Remove off of the heat and set to the side.

Preheat your griddle to 200-250°. Melt 1 tablespoon of butter per 4 slices of bread on the warm griddle, spreading it out to be the same size as the bread.

Lay down the 4 slices of bread on each melted butter section. I like toasting both sides of the bread for this grilled cheese. However you can make this the traditional way as well.

Once 2 slicea are golden on one side, use a spatula to lift it up and quickly melt a little more butter before flipping. So both sides of the bread are golden.

On these same two slices (the other two slices will go on last), top with 1 slice white cheddar, (warmed) leftover pot roast, a few spoonfuls of the pot roast gravy, divide the onions and top with the last slice of white cheddar.

When the last two slices are golden brown on one side, place the crispy side down on top of the cheddar so that the soft side is facing up. Carefully flip the sandwich and griddle, adding more butter as needed, until both sides are deeply golden brown and crisp.

Cut in half and serve.