1. Preheat oven to 350F. Set aside a 9×9 casserole dish. Break down rotisserie chicken, removing all skin, fat, bones and excess bits from the meat. Chop meat into 1/2-inch pieces. Place in a large bowl with the frozen peas. Set aside.

2. In a large saucepan over medium heat, heat olive oil until shimmering. Add celery, carrots, onion, garlic and thyme and cook, stirring frequently, until carrots are tender, 5-7 minutes. Stir in 1/4 teaspoon salt and 1/4 teaspoon pepper. Remove from pan and pour into the large bowl with chicken and frozen peas.

3. In the same, now empty, saucepan, melt the butter. Stir in the flour (mixture will be thick and dough-like) and cook, stirring constantly, for 1 minute. Gradually pour in the chicken stock while whisking constantly. Add the milk and cook until mixture has thickened slightly and coats the back of a spoon, about 7 minutes.

4. Pour into bowl with chicken, celery mixture and peas. Stir together and season with salt and pepper to taste.

5. Preheat oven to 350F. Pour entire mixture into casserole dish. Top the surface with the sheet of puff pastry – cutting into shapes and decorating the top if desired. If not, be sure to slice a few slits in the pastry to release steam. Bake for 45-60 minutes, until puff pastry is golden brown and filling is bubbling around the sides.