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Melt 2 tablespoons of the butter in a large pan. On low-medium heat, cook the onions until they soften and just start to caramelize. This will take about 6 to 8 minutes.
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Add the chopped spinach and cook until most of water is evaporated. The spinach will cook down. Add the salt and onion powder. This took about 7 to 8 minutes.
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Stir in the artichoke hearts. Cook for 3 additional minutes.
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Add the cream cheese. Cook until the cream cheese melts and you are able to stir it into the spinach. This will take about 2 minutes.
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Stir in the other cheeses (gruyere, mozzarella, and parmesan). Cook for about 1 additional minute.
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Remove the pan from the heat. Stir in the mayonnaise and sour cream.
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After the mixture comes to room temperature, stir in 2 lightly beaten eggs. At this point you can refrigerate the mixture until ready to use. You can make this part a day ahead.
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Preheat the oven to 400 degrees Fahrenheit.
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Unfold the puff pastry sheets on a wooden board. Divide each sheet into 5 x 5 inch squares.
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Generously grease each muffin tin with butter. Fit a puff pastry square into each tin.
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Divide the spinach mixture evenly among the tins. Fold the pastry inward.
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Brush the top of each pastry with the remaining (beaten) egg.
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Melt 2 tablespoons of butter and drizzle over each muffin.
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Bake in a preheated oven for about 22 minutes, or until the tops are golden.