• Chopped chives for garnish, optional
Preheat oven to 400 degrees, and butter six (8-ounce) ramekins or ovenproof custard cups. Place the ramekins or cups on a rimmed baking sheet.
Heat 1 tablespoon butter in a 12-inch nonstick skillet. Add the prosciutto and cook, stirring, until crispy, about 2 to 3 minutes. Use a slotted spoon to transfer prosciutto to a plate.
To the same skillet over medium heat, add remaining 1 tablespoon butter and shallot and cook, stirring, until soft, about 2 to 3 minutes. Add the spinach in batches, carefully stirring as it wilts. Keep adding more spinach when there is room in the skillet. Cook the spinach until it is all completely wilted. Add 1/2 cup cream, 1/4 teaspoon salt, pepper and nutmeg. Cook, stirring frequently, for 5 to 6 minutes, or until the cream is slightly thickened. Add the Parmesan cheese and cook for 1 minute more, until the cheese has melted into the cream. Remove from the heat and stir in the lemon juice. Taste for seasoning and divide among prepared ramekins.
Crack 1 egg into each ramekin. Spoon a teaspoon of cream over each yolk and season with a little of the remaining salt. Bake until the eggs are bubbly around the edges but not quite set in the middle, 10 to 13 minutes. (For firmer eggs, bake an additional 1 minute.) Remove from the oven immediately — the eggs will continue to set. Garnish with crispy prosciutto and chives, if desired, and serve warm.