Preheat the oven to 190 °C in good time.
Add sage and onion seasoning to the water, stir in butter and milk.
Mix with the mashed potatoes and pour into a greased ovenproof dish.
Press two indentations in the surface and carefully mold an egg into each.
Place tomato slices around the outside.
Bake the casserole in a heated oven at 190 °C for about 15-20 min.
Serve immediately.