preheat the oven to 400 degrees fahrenheit
put the potatoes in a pot on the stove and cover with cold water and bring to a boil on high heat. cook for 12-15 minutes, or until the potatoes are soft when pierced with a fork
drain the potatoes, then return to the pot. add the milk, butter, salt and pepper and mash until creamy
in a large oven-safe saute pan, heat the oil on medium heat. add the onion & garlic and saute for about a minute. add the diced carrots, diced celery, and ground beef. cook for 8-10 minutes or until the meat is browned. break up the meat while cooking with a spatula
drain the fat from the pan and add the broth, tomato paste, worcestershire sauce, rosemary, thyme, salt and pepper. Simmer for about 4-6 minutes, until the sauce has thickened
add the frozen peas & corn and stir together. turn off the stove and use the back of your spatula to flatten the meat mixture into a single layer
dollop the mashed potatoes on top of the meat and use a spoon or spatula to spread it flat to the edges
bake the cotage pie until slightly golden, about 25-30 minutes