In a small bowl, make the dressing: combine the mayonnaise, catsup, relishes, onion, garlic, lemon juice, and black pepper to taste; set aside.
Spread butter on one side of each slice of bread.
Heat a large non-stick pan over medium heat and place one slice of bread, butter-side down, in the pan.
Assemble the sandwich: on top of the bread, place the sliced pastrami; top meat with a couple of tablespoons of the dressing; then spread the sauerkraut over, add some more dressing, top with the cheese, then place the other slice of bread- buttered side up- on top (this arrangement helps keep the dressing from soaking the bread too much).
Cover the grilling sandwich with a lid (a Reverware saucepan lid works well for this) and let grill for 2 to 3 minutes, checking to make sure that the bread does not burn- the idea is to heat the air underneath the lid and melt the cheese easier as you're also browning the bread.
Carefully turn the sandwich, re-cover, and grill other side. (I say "carefully" because, depending on how much of the fillings you put in, this step can get a little messy -- ).
Cut the completed grilled sandwich in two and serve with the extra dressing for dipping, along with a dill pickle and potato chips or potato salad.