1.Preheat oven to 180°C (160°C fan-forced). Use 20g of the butter to generously grease four 2/3-cup (160ml) ovenproof dishes. Place dishes in a baking dish.
2.Heat remaining butter in a large non-stick frying pan. Add mushrooms, cook, stirring occasionally, over medium heat until golden brown. Add spinach, cook, a further 1 minute, or until the spinach is wilted. Divide the mixture between the dishes. Top with prosciutto.
3.Whisk eggs in a large bowl with sea salt and freshly ground black pepper. Pour egg into dishes, sprinkle with cheese. Pour boiling water into baking dish to come halfway up the sides of the dishes. Bake, about 20 minutes, or until egg is set. Grill tops to brown the cheese, if desired.
4.Serve with fingers of buttered toast, if you like.