First, make a quick Mornay sauce by sautéing a chopped onion with 2 tablespoons of butter.
Once the onions are translucent, add in 2 tablespoons of flour.
When the mixture thickens, add a cup of milk. As the milk begins to simmer, it will thicken.
Turn the heat down to low and add a cup of grated gruyere—stirring until it melts and is smooth.
To serve, lay thinly shaved pastrami on a thick slice of toasted sourdough, then pour the Mornay on top. Top with chopped chives and sprinkle with paprika.