In a cast iron skillet, melt the butter over medium-low heat and add the onions. Stir the onions about every 5 minutes for 45 minutes, until they are wilted and dark brown, nicely carmelized.
In microwave bowl or small saucepan, warm cream cheese, then stir until smooth.
Grill hot dogs until done.
Lightly toast buns, then smear cream cheese inside bun.
Put hot dog in bun, add onions, sauerkraut, and top with spicy brown mustard.
Recipe adapted from Sonofabfood.com