*sigh* Comfort food. Enough said.
3 pounds baking potatoes, peeled & sliced into 1″ rounds
1 tbsp vegetable oil
6 medium carrots, chopped
6 celery stalks, chopped
1 large onion
1/2 tsp dried thyme
1/4 cup all-purpose flour
1/4 cup tomato paste
1 pound ground beef
1 pound ground pork
1 cup whole milk
1 1/2 cups shredded aged cheddar cheese
Preheat oven to 450F. Place potatoes in a large saucepan, and cover by an inch of salted water. Bring to a boil; reduce to a simmer. Cook until potatoes are easily pierced with the tip of a paring knife, 15 to 20 minutes. Meanwhile, heat oil in a large Dutch oven or heavy pot over medium-high. Add carrots, celery, onion, and thyme. Cook, stirring occasionally, until vegetables are tender, 8 to 10 minutes. Add flour and tomato paste; cook, stirring, 1 minute. Add beef and pork; cook, stirring occasionally, until no longer pink. Add 1 cup of water and bring to boil, simmer 1 minute. Set filling aside.
Drain potatoes and return to pan. Cook over medium heat, stirring, until liquid has evaporated and a thin film covers bottom of pan, about 1 minute. Remove pan from heat; add milk and 1 cup of cheese. Mash until smooth; season with salt and pepper. Pour beef filling into a 13×9 inch baking dish. Drop dollops of potato-cheddar topping over filling; spread to edges with a spatula. Sprinkle with remaining 1/2 cup cheese. Bake until topping in browned and filling is bubbling rapidly, about 20 minutes. (You can make this up to a day ahead of time and refrigerate. Increase the baking time to 35 minutes if chilled). Let stand for 5 minutes before serving.