Preheat oven to 425°F (220°C).
Using a mandolin, slice the sweet potatoes to about 3-millimeter thick slices. In a large bowl, combine sliced sweet potatoes, oil, salt, and pepper. Lay out sweet potato slices on a large sheet pan and bake for 10 minutes.
Combine eggs, tomatoes, green onions, bell pepper, garlic powder, salt, pepper, and milk in a large bowl, whisking to combine.
Remove sweet potatoes from oven once edges start to crisp and turn the oven down to 400°F (200°C) . Line the sides and base of a 9x5-inch (20x10-cm) greased loaf pan with the sweet potato slices. Repeat with ham slices, then add spinach, another layer of sweet potatoes slices, the egg mixture, another layer of ham, and a final layer of sweet potato slices. Fold the outside sweet potato slices over the final layer of sweet potatoes. Bake for 30 minutes.
Place a plate over top and invert the loaf pan onto the plate, slice and serve.