Cover potatoes with cold water and 1 tablespoon of kosher salt in a large pot, and bring to a boil. Cook until potatoes are fork-tender, about 15-20 minutes.
While potatoes are cooking, sauté the diced onion in 2 tablespoons of the butter in a large skillet until softened (about 2 minutes). Add the ground beef and sauté until JUST fully cooked, breaking it up with a spoon into small pieces (about 5-7 minutes). Season with salt and pepper to taste. Don’t overcook it or it will be dry (see notes).
When potatoes are soft enough to pierce easily with a fork, drain, and mash with 4 tablespoons butter and the milk (3/4 cup). Season with salt and pepper to taste.
Layer ground beef mixture, frozen vegetables, and mashed potatoes in a 9×13 baking dish. Alternatively, divide the ingredients into two smaller baking dishes to freeze one (see notes).
Use the back of a spoon to smooth the potatoes, leaving some ridges, or use a fork to “rake” a ridged pattern into the potatoes.
Melt remaining tablespoon butter and drizzle evenly on top of potatoes.
Bake at 425° F for approximately 30 minutes, or until potatoes begin to brown. Run the broiler for the last 5 minutes for extra crispy potatoes.
