Preheat the oven to 400 degrees F. Line a baking sheet with parchment paper and lightly grease the paper. Set aside.
Place a 10-inch skillet over medium-high heat, melt the butter and add the garlic and spinach to the pan. Cook the garlic and spinach, using tongs to move the spinach from the bottom of the pan to the top, until the leaves are wilted and deep green in color, about 2 1/2 to 3 minutes. Season with the salt and pepper and set aside to cool.
Arrange 1 sheet of puff pastry on a lightly floured work surface. Use a rolling pin to roll out the puff pastry to 1/8-inch thickness. Use parchment or tracing paper to draw a fish silhouette that stretches 8-inches from head to tail, and 5-inches at its widest point. Cut the fish stencil and use it to cut out 4 puff pastry fishes. Repeat the process with the remaining sheet of puff pastry, making 4 more puff pastry fishes. Use the rolling pin to roll 4 of the fish another 1/4-inch thinner, and the other 4 pieces, 3/4-of an inch thinner.
Season each piece of flounder with the Essence, and place on top of 1 of the smaller pastry fishes. It may be necessary to cut the flounder in half lengthwise and shingle it over itself to fit on the puff pastry leaving at least a 1/4-inch space all around.
Once all 4 portions of fish have been placed on the pastry, divide the sauteed spinach among the 4 and top with the larger pieces of puff pastry. Fold and pinch the edges to make a seal. Use some scrap puff pastry to cut out an eye for the fish and affix it to the head using water as glue. Cut a gill into the head of the fish with out cutting all the way through the puff pastry.
In a small bowl, combine the egg with 1 tablespoon of water and whisk vigorously to combine. Place the fish on the prepared pan, and brush the egg wash over each of the fish. Bake until the puff pastry is golden brown, about 15 to 18 minutes. Let fish stand
for 5 minutes before serving with the Lemon Butter Sauce. Garnish with chopped parsley.