Using a knife, make a 1/8-inch slice lengthwise down each hotdog; set aside.

Heat bacon grease in a 12-inch cast iron skillet over medium-high heat. Add hot dogs, onion, tomatoes, jalapeno, garlic, and red pepper flakes to the skillet and cook until the vegetables have softened and everything is well browned, about 10 minutes, stirring occasionally.

Meanwhile, heat a second skillet over medium heat. Spread a thin layer of butter on the outside of each hotdog bun, and cook, turning occasionally, until lightly toasted on all sides.