Preheat the oven to 350°F.
After peeling and cubing the potatoes, give them a final rinse. Put the potatoes in a large saucepan or pot and cover with cold water. Add salt and bring to a boil. Reduce to a simmer, and cook the potatoes until soft (about 20 minutes). When they can be easily pierced with a knife, they're cooked.
Drain the potatoes and put them in a large bowl. Add 6 tablespoons of butter and 1/2 cup of milk and use a potato masher to mash your potatoes. Add additional milk to get a creamy, but not soupy consistency. Season with salt and pepper, and set aside.
Warm a cast-iron skillet over medium heat, add the tablespoon of olive oil, and cook the onion, stirring to just soften (about 2 minutes). Add the ground beef and stir to break apart. When there is just a little pinkness left in the meat, drain the fat from the skillet. Stir in the peas and corn kernels. Season with salt and pepper.
Spread the mashed potatoes over the meat and vegetables, distributing the potatoes evenly and smoothing the top. Cut the remaining 2 tablespoons of butter into slivers and dot the potatoes with them.
Cover with foil and bake for 30 minutes. Remove the foil and cook for another 10 minutes until the potatoes are lightly browned.
Allow to cool for 5 minutes before serving, and enjoy!