Preheat the oven to 400°F (200°C). Bring a large pot of salted water to a rolling boil for the pasta. Salt the water well (it should taste like the sea) because this is your primary opportunity to flavor the pasta itself; under-seasoned pasta makes the whole dish taste flat.

Cook the pasta 1–2 minutes less than package directions (about 7–8 minutes for most elbows/small shapes). You want the pasta slightly undercooked (al dente) because it will finish cooking in the oven and absorb some sauce — this prevents a mushy final texture.

While the pasta cooks, warm a large skillet over medium heat and add 1 tablespoon olive oil. Add the finely diced onion and a pinch of salt. Sauté until the onion is soft and starting to brown (6–8 minutes). Browning develops natural sweetness and adds depth that balances the rich cheese and beef broth.

In a medium saucepan melt 2 tablespoons of the butter over medium heat. Sprinkle in the flour and whisk constantly to form a smooth roux, cooking for 1–2 minutes until it smells slightly toasty but not nutty. This cooks out the raw flour taste and will thicken the sauce.

Slowly whisk in the milk, then the beef broth, pouring in a thin stream while whisking to prevent lumps. Bring the mixture to a gentle simmer — small bubbles at the edge — and cook 3–4 minutes until it thickens slightly. Using both milk and beef broth adds creaminess plus an umami backbone; simmering activates the starch in the roux so the sauce becomes velvety.

Lower the heat and stir in the Dijon mustard and smoked paprika (these lift and brighten the flavor). Gradually add 6 ounces of the shredded cheddar, one handful at a time, stirring until melted and smooth. Add salt and freshly ground pepper to taste. Adding cheese off the heat (or at low heat) prevents the proteins from seizing and keeps the sauce silky rather than grainy.

Drain the pasta, reserving 1/2 cup of the cooking water. Return pasta to the pot, stir in the sautéed onion, then pour the cheese-beef sauce over the pasta. Toss gently to coat, adding up to 1/4–1/2 cup reserved pasta water if the sauce is too thick — the starchy water loosens the sauce without watering down flavor.

Transfer the sauced pasta to a 9x9-inch or similar shallow baking dish. Sprinkle the remaining 2 ounces (about 1/2 cup) of shredded cheddar over the top to create pockets of melty cheese under the crust.

Make the topping: in a small skillet melt the remaining 2 tablespoons butter, stir in the panko breadcrumbs and cook 2–3 minutes until the crumbs are golden and crisp — toasting them in butter adds a nutty flavor and a crunchy contrast. Mix in the grated Parmesan and a pinch of smoked paprika, then scatter over the pasta.

Bake in the preheated oven 12–15 minutes until the topping is golden and the interior is bubbling. If you want extra color, broil 1–2 minutes at the end, watching closely to avoid burning. Baking melds the flavors and allows the sauce to set slightly so portions hold their shape on the plate.

Remove from oven and rest 5 minutes. The short rest lets the sauce thicken just enough so the dish isn’t runny but still creamy. Serve straight from the dish with a grind of fresh black pepper and an optional sprinkle of chopped parsley for brightness.