I don't make this very often because, honestly, it's artery clogging. When I do, no one is late for dinner. I love pastrami. I love burgers. I love pattymelts. And this one will fill you up to next year. Great on a cold winter night with homemade tomato soup too.
Melt 4 tablespoons of butter in a saute pan over medium heat and add the onions. Season with salt and freshly whacked black peppercorns and saute the sliced onions slowly until a medium dark, golden brown. About 20 minutes. Don't worry about the onions being overwhelmed by the whacked black peppercorns. It's not hot, just tangy. Remove the onions to a platter.
Form the ground beef into 4 patties the same size as the sourdough bread, and season with salt and pepper. Saute until done to your preference in the same pan as the onions.
In the same pan, add the pastrami and saute until very hot.
Heat an electric skillet and add 2 tablespoons of butter. With the remaining butter, butter one side of each 8 slices of the sourdough bread and put into the electric skillet. Brush the other side of the bread with the coarse grained mustard. Top each slice of bread with one slice of the prepackaged Velveeta.
Top four slices of bread with the prepared burger, 1/4 of the hot pastrami, and 1/4 of the prepared onions. Put the other slice of bread on top of the melt and press slightly.
Saute until the cheese starts to melt and the underside is a nice golden brown. Turn carefully and saute until the underside is a nice golden brown. You might have to add more butter. Warning, the cheese will probably melt all over the place.