01 - Preheat oven to 375°F. Lightly coat a 9x13-inch baking dish with nonstick cooking spray and set aside.
02 - Place the peeled and cubed potatoes in a large pot of salted water. Bring to a rolling boil and cook for 15 to 20 minutes until fork-tender. Drain thoroughly, return to the pot, and mash together with the milk and butter until smooth and creamy. Season with salt and pepper to taste, then set aside.
03 - While the potatoes cook, heat a skillet over medium heat. Add the diced onion and minced garlic, sautéing for 3 to 4 minutes until softened and fragrant. Remove from heat and let cool slightly.
04 - In a large mixing bowl, combine the ground beef, ground pork, sautéed onion and garlic, breadcrumbs, eggs, 1/4 cup ketchup, Worcestershire sauce, salt, and pepper. Mix gently with your hands until just combined, being careful not to overwork the meat.
05 - Press the meat mixture evenly into the bottom of the prepared baking dish, creating a uniform layer. Spread the frozen mixed vegetables over the meat in an even layer.
06 - Spread the mashed potatoes over the vegetable layer, smoothing the surface with a spatula to create an even, level topping.
07 - Drizzle additional ketchup evenly across the mashed potato surface, then sprinkle the shredded cheddar cheese over the top.
08 - Bake uncovered on the center rack for 35 to 40 minutes, until the casserole is bubbling around the edges and the cheese is melted and golden brown.
09 - Remove from the oven and let the casserole rest for 10 minutes to allow the layers to set before slicing into portions and serving.