Spray the sides and back of a 9x13 casserole. Lay 1/2 of the spinach leaves in a bed on the bottom of the casserole, coming up the sides. Place the corned beef in the middle, and break apart with a fork. Spread the beef around as much as possible. Throw in the diced tomatoes, evenly, and then sprinkle the sliced onions over the top too. Next, dot with the mayonaise, and cover with remaining spinach leaves. Pour coconut milk over all. Cover tightly with foil, shiny side down, and bake for about 1 hour.