Preheat oven to 350 degrees. In a large pot of boiling salted water, cook pasta
3 minutes less than package instructions. Add broccoli and cook 3 minutes more. Drain.
Meanwhile, heat oil in a large pan over medium-high. Add onion, 1/2 teaspoon salt, and ground mustard. Saute until onion is translucent, about 4 minutes. Add flour
and cook, stirring, 1 minute. Add Neufchatel, 1 tablespoon at a time, stirring until incorporated. Gradually whisk in milk, then bring to a boil. Reduce heat and simmer, about 3 minutes, stirring constantly. Stir in cheddar and Parmesan, then add pasta and broccoli. Season with salt and pepper.
Transfer to an 8-inch square baking dish
and bake until bubbling in center, about 25 minutes. Sprinkle with panko, and serve.