Bring a pot of water to a boil, add chicken breasts, and simmer until fully cooked through, about 30 minutes, then remove the chicken and shred it. Reserve the cooking broth.
2 boneless skinless chicken breasts
Heat a non-stick skillet over medium-high heat. Add olive oil and butter, allowing them to melt together.
1 tablespoon olive oil, 1 tablespoon butter
Add the onion and whole garlic cloves. Sauté for 5-7 minutes until softened.
1 onion, 2 garlic cloves
Add the celery, carrots, and peas. Cook for a few more minutes.
1 cup celery, 1 cup carrots, 1 cup frozen peas
Sprinkle in the flour and mix well to form a roux. Slowly whisk in the heavy cream until fully incorporated and smooth. Add the shredded chicken and stir it. Remove from heat and season with thyme, salt, and pepper. If needed, adjust the consistency by adding some of the reserved chicken broth.
2 tablespoons all-purpose flour, 1 cup chicken broth, 1 ½ cups heavy cream, 2 teaspoons thyme, Salt and pepper
Preheat the oven to 180ºC (355ºF). Grease a pie dish with cooking spray and line it with one sheet of pastry. Add the cooked chicken filling, then top with the remaining pastry sheet. Trim off any excess dough and crimp the edges to seal.
2 sheets puff pastry
Make a small slit in the center of the pie top to allow steam to escape. Bake for 15-20 minutes, until the pastry is golden brown.
Remove from the oven and garnish with fresh thyme sprigs before serving.
Thyme springs