Preheat and prep: Preheat oven to 375°F (190°C). Bring a large pot of salted water to a boil for the pasta — adding salt to the water seasons the pasta internally as it cooks. While water heats, pat the chicken dry and season both sides with 1/2 tsp kosher salt and 1/4 tsp black pepper.

Cook pasta al dente: Add the 12 oz (340 g) dry pasta to the boiling water and cook 1–2 minutes less than package directions so it’s very slightly underdone (usually 8–9 minutes for penne). The pasta will finish cooking in the oven; undercooking prevents a mushy casserole.

Sear the chicken: While pasta cooks, heat 1 tbsp olive oil in a large skillet over medium-high heat until shimmering. Add the chicken and sear 2–3 minutes per side until deeply golden but not fully cooked through. Browning (Maillard reaction) builds savory flavor base that will lift the whole casserole. Transfer chicken to a cutting board to rest for 5 minutes, then slice or dice into bite-size pieces.

Caramelize the onion: Reduce skillet heat to medium and add 1 tbsp butter and the sliced onion. Cook, stirring occasionally, 12–15 minutes until the onion turns deep golden and sweet. Caramelization concentrates sugars and adds complex sweetness that balances the rich cheese sauce. If the onions begin to stick, add a splash of water to deglaze and scrape up the browned bits.

Add garlic and finish onions: Stir in the minced garlic and remaining 1 tbsp olive oil; cook 30–45 seconds until fragrant. Garlic browns quickly — adding it after onions prevents burning and gives fresh aromatic lift.

Make the cheddar béchamel: In a medium saucepan over medium heat melt the remaining 2 tbsp butter. Whisk in 3 tbsp flour and cook for 1–2 minutes to form a blond roux; this removes raw flour taste and thickens the sauce. Gradually whisk in 2 cups whole milk in a thin stream to avoid lumps, then simmer gently until the sauce thickens and coats the back of a spoon (about 5–6 minutes). Taste and season with 1/2 tsp kosher salt, 1/4 tsp black pepper, and 1/8 tsp nutmeg (nutmeg brightens creamy sauces).

Incorporate the cheese: Remove sauce from the heat and add 1 3/4 cups (about 195 g) of the shredded cheddar (reserve 1/4 cup for the filling and 1/2 cup for the topping mix). Whisk until just melted and smooth — adding cheese off heat prevents oily, grainy separation. If sauce becomes too thick, whisk in 1–2 tbsp warm milk to smooth.

Assemble the casserole: Drain pasta and return to the pot. Add the seared chicken pieces, caramelized onions with any pan juices, the cheesy béchamel, and the reserved 1/4 cup shredded cheddar. Stir gently to combine — the goal is an even coating without crushing the pasta. Adjust salt and pepper to taste.

Transfer to baking dish and top: Spoon the mixture into a 9x13-inch (23x33 cm) casserole dish. Sprinkle the remaining 1/2 cup shredded cheddar and the 1/2 cup grated Parmesan evenly over the top to create a golden crust while baking. The combination of cheddar and Parmesan gives both melty stretch and a nutty, browned flavor.

Bake to finish: Bake uncovered for 18–22 minutes until the top is bubbling and deep golden. If you like an extra-crispy top, broil 1–2 minutes at the end — watch carefully to avoid burning. Baking melds flavors and gives the casserole a pleasing set and a caramelized crust.

Rest and serve: Remove from the oven and let rest 8–10 minutes before scooping. Resting lets the sauce thicken slightly for cleaner portions and allows flavors to settle. Serve garnished with an extra crack of black pepper or chopped fresh parsley if available.