Place oil in a cast iron skillet over medium heat. Once hot, add the minced onion and cook for 2 minutes until soft and lightly golden.
Remove onion to a small bowl. Add the ground beef to the skillet and cook until browned throughout, about 8 minutes.
Once the beef is fully cooked, return the cooked onions to the skillet and add the minced garlic.
Cook for 2 minutes, then stir in the crushed tomatoes and drained beans. Mix well to combine. Add all the seasonings and stir to distribute evenly.
Continue cooking while you prepare the mashed potatoes.
Preheat oven to 375°F. Allow the mashed potatoes to cool slightly, then transfer into a piping bag fitted with a large star tip.
Remove the cast iron pan from the heat. Pipe dollops of mashed potatoes on top of the chili, covering it completely.
Bake for 15-20 minutes until potatoes are golden brown.
Notes
Use instant mashed potatoes for quick prep. No piping bag? Use a ziplock with corner cut off or spoon dollops. Cover chili completely with potatoes to prevent bubbling over. Let rest 5-10 minutes before serving.