Boil pasta and broccoli:
In a large saucepan, bring salted water to a boil. Add pasta and cook 2 minutes less than package directions. Add broccoli in the last 2 minutes of cooking. Drain and set aside.
Make the cheese sauce:
In another saucepan, melt butter over medium heat. Whisk in flour and cook 1–2 minutes to form a roux.
Slowly whisk in warm milk. Cook, whisking constantly, until thickened (5–7 minutes).
Reduce heat to low and stir in cheddar, Gruyère, Dijon (if using), and seasonings. Mix until smooth. Taste and adjust salt.
Combine:
In your stainless steel skillet, combine drained pasta and broccoli with the cheese sauce. Stir to coat everything evenly.
Make the topping:
In a small bowl, mix panko, melted butter, Parmigiano, and optional herbs or chili flakes.
Assemble and bake:
Sprinkle the topping evenly over the pasta mixture in the skillet.
Bake at 400°F (200°C) for 15–20 minutes until bubbly and golden on top. Broil for 1–2 minutes at the end for extra crispness if desired.
Serve:
Let rest 5 minutes before serving directly from the skillet. Garnish with fresh cracked pepper or chopped parsley if desired.