Heat oven to 450° F.
In a large skillet over medium heat, combine the bacon and onion. Cook until the onion is caramelized and golden brown, 9 to 10 minutes. Remove from heat, mix in the spinach, and set aside.
Meanwhile, in a small bowl, whisk together the milk, egg, salt, and oregano. Set aside.
Coat a 9-by-13-inch baking dish with vegetable spray. Arrange 1 layer of potatoes, overlapping slightly. Spread the tomatoes evenly on top of the potatoes. Pour 1/3 of the milk mixture over the tomatoes. Add another layer of potatoes and then the bacon-spinach mixture, spreading evenly. Top with another third of the milk mixture. Finish with the last layer of potatoes and sprinkle on the shredded cheese. Drizzle the remaining milk mixture over the dish.
Cover with foil and bake for 45 minutes. Remove the foil and bake another 10 minutes or until the cheese is golden brown. Remove from oven and let rest, covered, for 10 minutes before serving.
If You Don't Have... canned tomatoes, use a jar of spaghetti sauce.