Preheat the oven to 400°F (200°C). Butter or oil a 9x13-inch (23x33 cm) baking dish. Preheating ensures the casserole starts baking immediately when it goes into the oven so the topping crisps evenly.

Bring a large pot of salted water to a boil and cook the pasta for 2 minutes less than the package's al dente time (usually 8–9 minutes for penne). Drain and toss immediately with a drizzle of olive oil to prevent sticking. Undercooking by a couple minutes keeps the pasta from going mushy during the second bake.

Season the chicken breasts with 1/2 tsp salt and a pinch of pepper. In a skillet over medium-high heat, add 1 tsp olive oil and sear the breasts 2–3 minutes per side until golden — you're not trying to fully cook them here, just to color and lock flavor. Then transfer to the simmering pasta water for 8–10 minutes to finish cooking gently (or poach in a separate pot of simmering water). Poaching keeps the chicken tender and prevents drying.

When chicken is cooked through (internal temperature 165°F / 74°C), remove to a cutting board, let rest 5 minutes, then shred or dice into bite-sized pieces. Resting allows juices to redistribute so the meat stays moist.

Meanwhile, in the same skillet, warm 1 tbsp olive oil over medium heat and sauté the diced onion until translucent and just starting to brown, about 5–6 minutes. Add the minced garlic and cook 30 seconds until fragrant. Sautéing the onion and garlic builds a savory foundation for the sauce.

Lower the heat and add the 2 tbsp butter to the skillet; once melted, sprinkle in the 2 tbsp flour and stir constantly for 1 minute to make a roux. Toasting the flour cooks out raw starch and gives the sauce a nutty backbone.

Slowly whisk in the 2 cups of cold whole milk a little at a time so the roux absorbs it smoothly. Bring to a gentle simmer and cook until the sauce thickens to coat the back of a spoon, about 4–5 minutes. A gentle simmer develops a silky béchamel without curdling the milk.

Remove the sauce from heat and stir in 6 oz (3/4 of the shredded cheese), Dijon mustard, smoked paprika, remaining 1/2 tsp salt, and 1/4 tsp pepper until fully melted and homogenous. Seasoning the sauce before adding the chicken and pasta ensures balanced flavor throughout the casserole.

In a large mixing bowl, combine the cooked pasta, shredded chicken, and the cheese sauce. Taste and adjust seasoning. The bowl lets you fold ingredients gently so you don't break the pasta or shred the chicken too fine.

Spoon the mixture into the prepared baking dish, smoothing the top. Sprinkle the remaining 2 oz (1/4 of shredded cheese) evenly over the surface.

In a small skillet, melt 1 tbsp butter and toss with the 1/2 cup breadcrumbs and a pinch of smoked paprika and salt. Toast for 2–3 minutes until golden — toasting the crumbs in butter gives the topping flavor and crispy texture that contrasts the creamy interior.

Scatter the toasted breadcrumbs over the casserole and bake at 400°F (200°C) for 18–22 minutes, until the top is golden and the sauce is bubbling around the edges. Baking melds flavors and finishes cooking the pasta while crisping the topping.

If you want extra browning, switch the oven to broil on high for 1–2 minutes watching carefully so the crumbs don't burn. Remove, let rest 5 minutes to set, then garnish with chopped parsley and serve. Resting lets the sauce thicken slightly so the casserole slices cleanly.