Heat the butter and olive oil in a large cast iron skillet over medium-high heat. Add the onion, carrots, and garlic, and cook until softened, about 5 minutes. Add the ground beef and season with salt, breaking up, until browned, about 10 minutes. Mix in the flour, stock, thyme and peas into the mixture. Pipe mashed potatoes over top.
4 tabelspoons Butter Parsley to finish
Bake until mashed potatoes are golden brown in parts, about 30 minutes. Mash together with butter. Let cool slightly before serving. Finish with parsley.