Cook penne until just shy of al dente—still with a bit of bite—since it will finish cooking in the oven.
Drain and set aside. Toss with a tiny pat of butter or drizzle of oil if you’re worried about sticking.
Step 2: Sauté the veggies
In a large deep skillet or Dutch oven, melt butter over medium heat.
Add diced onion, carrots, and celery. Sauté for 5–7 minutes, stirring occasionally, until the vegetables are softened and starting to turn golden at the edges.
Stir in minced garlic and cook for about 1 minute, just until fragrant.
Step 3: Make the creamy sauce
Sprinkle flour evenly over the vegetables and stir to coat, forming a roux. Cook for 1–2 minutes to remove the raw flour taste.
Gradually whisk in chicken broth, stirring constantly to avoid lumps, then whisk in the milk.
Bring to a gentle simmer, stirring frequently, until the sauce thickens enough to coat the back of a spoon. This step is key so the finished pasta isn’t runny.
Step 4: Add chicken and peas
Stir in cooked, shredded or diced chicken and frozen peas.
Season with dried thyme, salt, and pepper to taste. Let the mixture bubble for a few minutes so flavors meld and the peas heat through.
Step 5: Combine with pasta
Add the drained penne to the skillet and fold gently until every piece is coated in the creamy sauce and the chicken and veggies are evenly distributed. If the mixture seems very thick, you can loosen it with a splash more broth or milk.
Step 6: Transfer and top with puff pastry
Preheat the oven to 375°F (190°C). Grease a 9×13-inch baking dish.
Pour the pasta mixture into the prepared dish and spread into an even layer.
On a lightly floured surface, unfold the thawed puff pastry and gently roll it out to roughly match the size of your baking dish.
Lay the puff pastry over the pasta, trimming edges if needed and tucking them slightly inside the dish. Cut a few small slits in the top to vent steam.
Step 7: Egg wash and bake
Beat the egg with a splash of water to make an egg wash. Brush it evenly over the puff pastry surface.
Bake at 375°F for 25–30 minutes, or until the pastry is puffed and deep golden and the filling is bubbly around the edges.
Step 8: Rest and serve
Let the casserole rest for 5–10 minutes before serving. This allows the sauce to settle and makes scooping portions cleaner.
Garnish with chopped fresh parsley or thyme, then spoon into bowls, making sure each serving gets some flaky pastry top.
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