Preheat the oven to 400° Fahrenheit. Prepare a 9x13 inch baking dish by lightly spraying with non-stick cooking spray. Set aside.
In a 3 quart saucepan, melt the 4 Tablespoons unsalted butter over medium heat. Sauté 1 small yellow onion and 1 12 ounce bag frozen mixed vegetables until the onion is translucent and vegetables are tender, about 5 minutes. Reduce heat to medium-low and stir in the 3 teaspoons minced garlic. Cook for 1 minute.
Add the ⅓ cup all-purpose flour, 1 teaspoon fine sea salt, 1 teaspoon ground black pepper, ½ teaspoon dried basil, ½ teaspoon dried thyme, ½ teaspoon dried rosemary, and ¼ teaspoon smoked paprika and stir to combine.
Stir in the 1 ½ cups chicken stock and 1 ¼ cups heavy cream. Continue to cook, stirring occasionally, until the mixture thickens, about 5-7 minutes.
Stir in the 1 pound boneless skinless chicken breastand 1 cup parmesan cheese. Spread the filling mixture into the prepared pan.
Cut the thawed sheet of 1 frozen puff pastry sheet into squares and place them on top of the filling mixture, making sure to cover the filling entirely.
Use a silicone brush or pastry brush to brush 1 egg over the top of the puff pastry. Bake for 40-45 minutes or until the puff pastry is golden brown and the filling is hot and bubbly.Let sit for 10-15 minutes before serving.