Add the oil to a large skillet and place it over medium-high heat for 2 min. Add the onions and cook for 5 min stirring occasionally.
Add the ground beef to the skillet and brake it apart with a wooden spoon. Add the parsley, rosemary, thyme, salt, pepper and stir well. Cook until the meet is browned stirring occasionally – about 8 min, then add the Worcestershire sauce and garlic. Stir to combine and cook for 1 min more. Add the flour and tomato paste, stir until well incorporated and no clumps of tomato paste remain.
Add the broth, frozen peas, carrots and corn. Bring the liquid to a boil, then reduce the temperature a bit and simmer for 5 min stirring occasionally.
Set the meat mixture aside and preheat the oven to 400ºF / 200ºC.
To make the potato topping place the potatoes in a large pot and cover them with water. Bring to a boil and reduce to simmer. Cook until the potatoes are fork tender – about 10 to 15 min.
Drain the potato in a colander and return to the hot pot and let them rest for 1 min to evaporate any remaining liquid.
Add butter, milk, garlic, salt, pepper and mash the potatoes until all ingredients are mix together. Add the parmesan cheese if desired and stir again until is well combined.
Assemble the casserole by pouting the meat mixture in a lasagna dish or baking dish. Spread it out into an even layer and spoon the mash potatoes on the top of the meat spreading them carefully into an even layer.
Bake uncovered for 25 to 30 min and cool it for 15 min before serving.
