Heat the milk in a pot over medium heat until it just starts to bubble, but is not boiling, 3-4 minutes. Remove from heat. Heat the butter over medium heat in a separate, heavy bottomed pot. When the butter has just melted, add the flour and whisk constantly until the mixture turns light brown, about 3 minutes. Remove from the heat.

Slowly pour the warm milk, about 1 cup at a time, into the butter-flour mixture, whisking constantly. It will get thick when you first add the milk, and thinner as you slowly pour in the entire 3 cups. This is normal. Once all the milk has been added, set the pot back over medium-high heat, and continue to whisk constantly. In the next 2-3 minutes the sauce should come together and thicken. Use the spoon test to make sure it’s ready. Dip a metal spoon into the sauce, if the sauce coats the spoon and doesn’t slide off like milk, then it’s ready. Add the salt and stir to combine.

Preheat your oven to 200 C. Meanwhile, cook the pasta in boiling water until tender, drain, rinse with cold water, and drain again. Then mash together the ginger and butter in a small bowl.

Add the sauce, cheese, and ginger butter to a large heavy-bottomed pot and cook over medium heat. Stir until the cheese is barely melted, about 3 minutes. Add the Sriracha and the cooked pasta and continue cooking while stirring continuously until the dish is nice and hot, another 5 minutes, then add the green onions and stir to fully combine.

Pour the mac into a 13x9 inch casserole pan and sprinkle with panko and some additional cheese, bake until hot and bubbly and the topping is golden, about 20 minutes. Remove from the oven and drizzle with more sriracha. Spoon into bowls and serve.