Chicken Pot Pie with a Roasted Tomatillo Sauce - Rick Bayless

Ingredients

• 1 pound tomatillos, husked removed and washed
• 2 to 3 fresh serrano chiles, stems discarded
• 4 garlic cloves, unpeeled
• 1 1/2 tablespoons olive oil
• 2 cups diced white onion
• Salt
• 8 ounces thinly sliced mushrooms ( I like shiitakes)
• 4 tablespoons unsalted butter, cubed
• 1/2 cup all-purpose flour
• 2 cups milk
• 1 rotisserie chicken, meat removed from the bones and diced
• 1 10-ounce package frozen chopped spinach, defrosted and squeezed dry
• 1 cup frozen corn kernels
• 1 piece frozen puff pastry, defrosted
• 1 egg plus 1 tablespoon water for egg wash