a small stainless steel skillet, toast caraway and cumin seeds until fragrant. Grind in a spice grinder or with a mortar and pestle, and reserve.
Heat a cast-iron or 2-burner griddle or a large, cast-iron skillet over medium-high heat.
Place beef in a mixing bowl and add ground caraway and cumin. Douse evenly with Worcestershire and, using a handheld grater, grate onion directly over bowl to catch juices. Season meat generously with Kosher salt and pepper and mix with your hands to combine. Form 4 large, thin patties that are even thinner at the center for even cooking.
In a medium-size mixing bowl, combine sour cream, relish, ketchup and hot sauce, to taste. Add sauerkraut and mix well.
Drizzle hot griddle or pan with oil and cook patties 4 minutes on each side, remove and wipe griddle or pan clean. Reduce heat to medium.
Butter 1 side of each bread slice lightly and evenly. Build sandwiches with buttered slices facing out: bread, pickles, provolone, patty, sauerkraut and Swiss.
Griddle sandwiches until deeply golden and crispy, cut in half on a bias and serve.