Step 1: Cook the sweet potatoes and potato - Peel the sweet potatoes and the potato, then cut them into large chunks. Bring a pot of salted water to a boil and cook the tubers for about 15 to 20 minutes until they are fork tender. Drain well and let the steam escape for a minute so the mash does not turn watery.
Step 2: Mash the potatoes - Add the milk, butter, salt, pepper, and nutmeg to the cooked potatoes. Mash until smooth and creamy. You can use a hand masher for a slightly rustic texture or blend for an extra smooth finish. Set the mash aside and give yourself a little pat on the back for already being halfway there.
Step 3: Cook the ground beef - Heat a large skillet over medium heat. Add the ground beef along with the salt, garlic powder, paprika, and chili flakes. Cook for about 5 to 6 minutes, breaking the meat up as it browns. You are building the flavor right here, so take a moment to really let those spices wake up.
Step 4: Add the vegetables - Stir in the diced onion and red bell pepper. Cook for another 5 to 7 minutes until the vegetables are soft and the beef is fully cooked. The kitchen should be smelling pretty good at this point. Add the white miso and stir until everything is evenly coated and savory.
Step 5: Assemble the casserole - Preheat your oven to 200 C or 400 F. Spread the beef and vegetable mixture evenly in a 6x10 inch baking dish or similar size. Sprinkle a small amount of mozzarella over the beef. Spoon the mashed sweet potato mixture over the top and gently spread it into an even layer. Finish with the remaining mozzarella cheese.
Step 6: Bake until bubbly - Place the dish in the oven and bake for 15 to 20 minutes until the cheese is melted, golden, and bubbling around the edges. Let it rest for a few minutes before serving so the layers can settle a bit.