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1
Set a medium skillet over medium heat and add the butter. When the butter is hot, add the onion. Season the onion with salt and freshly ground black pepper. Stir the onion regularly until it starts to become soft and translucent. Reduce the heat to medium-low and stir occasionally, until the onions are soft, sweet, and slightly caramelized (approximately 30 minutes).
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2
While the onions are cooking, set a medium pot filled with water over high heat. Bring the water to a boil, then reduce the heat so that the water is simmering. Gently lower the eggs into the water (I use a mesh strainer to do this). Cook the eggs for exactly 8 minutes. Transfer the eggs to an ice bath to cool. Peel the eggs and store them in the refrigerator.
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3
By this time, the onions should be ready. Transfer them to a blender and add the water. Blend until a puree forms. Add the cream and lemon zest, and blend until they are fully incorporated. Season with salt and black pepper. Taste the sauce, and adjust the flavor as necessary (it should taste sweet and savory from the caramelized onions, and slightly acidic from the lemon zest). Set the sauce aside. Both the eggs and the onion sauce can be made a day or so ahead of time (the eggs, a few days ahead) and stored in the refrigerator until you are ready to make the crostini.
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4
Preheat the oven to 425u00b0 F. While the oven is heating up, chop the Swiss chard leaves into bite-size pieces. Set a large skillet over medium heat. Add enough olive oil to barely cover the surface of the skillet. When the oil is hot, add the Swiss chard. Cook, stirring regularly, until the chard is soft and wilted. Remove from the heat and season with salt, pepper, and the lemon juice. Set the chard aside. Drizzle the bread with olive oil, set the slices on a baking sheet, and cook them in the oven for approximately 4 minutes, until they are beginning to look brown and toasted.
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5
Set the oven to broil. Line a baking sheet with aluminum foil, and place the slices of bread on the baking sheet. Assemble the crostini by adding a layer of Swiss chard to each piece of bread. Slice the eggs (from top to bottom, so every piece has a bit of white and yolk), and place the egg slices on top of the Swiss chard in a single layer. Drizzle 1 1/2 tablespoons of the caramelized onion sauce over the top of each crostini (you will have extra sauce left over-store it in your refrigerator and use it with pasta for a nice snack on another day). Top each piece of bread with a handful of grated Gruyere. Broil the crostini until the cheese begins to bubble and turn brown. Keep a close eye on the crostini, as they will cook quickly. When they look beautiful, remove them from the oven, serve, and enjoy.
