Adjust oven rack to lower middle position and heat oven to 425 degrees F. Bring a generous 2 quarts of water and 1 tablespoon of table salt to boil in a large pot. Using back of the box cooking times as a guide, add pasta to the water, stirring several times at the beginning to prevent sticking. Add broccoli the final minute of cooking and then return water to a simmer. Setting a bowl underneath to catch the cooking liquid, drain pasta. Turn pasta and broccoli, along with half the scallion greens and half of the cheese, into a 13- by 9-inch oven-proof baking dish while making the sauce.

While pasta cooks, measure sauce ingredients. Heat 4 tablespoons of the butter in a large saucepan or small Dutch oven. Add scallion whites; sauté until softened, 2 to 3 minutes. Whisk in flour, then coconut milk and 1 cup of the pasta cooking liquid; whisk until thick and smooth. Whisk in mustard, nutritional yeast, turmeric, paprika. Remove from heat. Whisk in miso and remaining half of the cheese. Adjust seasonings, including salt and pepper to taste.

In a small bowl, mix panko, Parmesan, remaining scallion greens and butter, along with a sprinkling of salt and pepper. Pour sauce over pasta; toss to coat. Sprinkle with panko and bake until sauce is bubbly and crumbs are an impressive golden brown, about 20 minutes. Let cool for a few minutes and serve.