1.Preheat oven to moderate, 180°C. Lightly grease a 20cm round cake pan. Line base and side with baking paper.
2.In a bowl, whisk together eggs, cream, milk and mustard. Season to taste.
3.Dip potato slices into egg mixture and arrange a third over base of prepared pan. Top with half of corned beef.
4.Combine spinach, cheese and onion. Spread half over beef. Repeat layers, finishing with potato. Pour egg mixture over. Press firmly to submerge potatoes.
5.Cover with foil. Bake for 1 hour or until potatoes are tender. Uncover and bake another 5-10 minutes until golden and firm. Cool in pan for 15 minutes.
6.Invert pan onto a plate and remove paper. Turn over. Serve with salad.