1. Heat the oven to 450°. Put the bread on a baking sheet and toast in the oven, turning once, until lightly browned, about 5 minutes in all. Transfer to an oiled baking dish.
2. Meanwhile, bring a medium pot of salted water to a boil. Add the asparagus and cook until tender, about 5 minutes. Drain, rinse with cold water, and drain thoroughly.
3. Using your fingers, make a depression in the center of each slice of bread. Arrange one or two slices of prosciutto around the edge of each slice of bread. Reserve 2 tablespoons of the cheese. Press a quarter of the remaining cheese into the center of each slice of bread. Top the cheese with the spinach, and then surround that with the asparagus. Break an egg into a small dish. Carefully slip the egg into one of the spinach nests. Repeat with remaining eggs. Sprinkle the eggs with the salt and pepper and the reserved cheese. Bake, covered, until the egg whites are just set, 10 to 15 minutes.
Wine Recommendation: New Zealand sauvignon blancs have a distinctive style, with a powerful streak of acidity, ripe citrus and gooseberry flavors, and a singular note of asparagus. Try one with this egg dish for an intriguing pas de deux.