Lay the tomatillos, serranos, and garlic on a baking sheet lined with foil and place 4 inches below a very hot broiler. When the tomatillos blister, blacken and soften on one side, about 7 minutes, turn them over and roast the other side, 6 to 7 minutes more. Keep an eye on the garlic and serranos since they'll take less time to roast and you'll need to remove them sooner. Scrape the tomatillos along with their juices into a blender or food processor. Add the peeled garlic and serranos, process until smooth.
Heat a large dutch oven or 6 quart stockpot over medium-high heat. Add the olive oil, once it's hot, scoop in the onions and 1/2 teaspoon salt. Cook for about 10 minutes, stirring occasionally, until the onions are golden brown. Stir in the sliced mushrooms, continue cooking for an additional 5 minutes or until the mushrooms have softened and most of their liquid has evaporated. Reduce the heat to medium and add the butter. Once it's melted, stir in flour and continue stirring for 2 minutes to cook the flour. Add half of the milk whisking constantly to ensure that there are no lumps. Pour in the remaining milk and continue stirring until the mixture has come to a boil and the sauce has thickened.
Remove the pot from the heat and stir in the tomatillo puree. Once the tomatillo puree has been incorporated, mix in the chicken, spinach and corn. Taste and adjust the seasonings, if necessary. Scrape the filling into a 13 x 9-inch casserole dish. Let the filling cool completely, cover, and refrigerate overnight..
Roll out the puff pastry into a 15 x 9-inch rectangle and cut into 6 equal pieces. Place the pieces onto a baking sheet lined with parchment. Wrap tightly and refrigerate overnight.
The next day, preheat oven to 400 degrees. Mix the egg with 1 tablespoon water and brush the tops of the puff pastry pieces. Place the baking sheet on the upper third shelf and the uncovered casserole dish on the center shelf. Bake for 25 minutes or until the pastry is golden brown and the chicken filling is hot. Spoon the filling into bowls and top each serving with a piece of puff pastry. Serve immediately.