Make filling: In a wide and shallow pan over medium heat, add vegetable oil. Sweat onions, carrots, and garlic and season with salt.
When the vegetables are translucent, take out and set aside. Clean pan out and put back onto the stove. Get the pan hot and add enough vegetable oil to barely cover the bottom of the pan. Add the ground beef evenly and sear until crispy. Season with salt and pepper.
When beef is seared, add chicken stock. Bring to a boil, and reduce by half. Add remaining ingredients. Add Sriracha, and salt to personal liking. Set aside until assembly.
Make the topping: In a pot, add potatoes, garlic, thyme and rosemary. Cover with cold water and bring to a boil. When potatoes are tender, drain excess water. Discard rosemary and thyme.
Put potatoes back in the same pot, and back onto the stove. Cook remaining liquid out of the pot and stir to assure it doesn't burn. Add cream, salt, and beef fat. Mash potatoes with a "potato masher," or a food mill. Taste and add additional salt and beef fat if needed.
To assemble: In a small baking pan, layer potatoes and filling. Continue until there's no more filling--just be sure to leave enough potatoes to cover the top. When ready, put the pan under the broiler until browned. Serve.