1Cook potatoes in a large pot of boiling water until tender, about 20 minutes. Drain well. Add milk and salt. Mash until smooth. Set aside.
2In a large bowl, combine beef, bread crumbs, onions, egg white, ketchup, garlic, and black pepper. Mix well (using your hands works best.) Pat beef mixture over bottom and up sides of a 9" pie plate.
3Layer tomato slices over beef. Sprinkle oregano over tomatoes. Spread Swiss and feta cheeses over tomatoes. Top cheese with chopped spinach. Spoon mashed potatoes over spinach. Spread evenly, leaving a 1" border around edges. Meat and potatoes should not touch. Smooth top.
4Bake at 350 degrees for 45 minutes. Let cool 5 minutes. Slice into wedges and serve.