Preheat oven to 400°F.
Add pasta to salted, boiling water. Cook 2 minutes less than package directions. Drain and set aside.
In sauté pan, over medium heat, cook pancetta until brown and crispy. Add green onions and cook 2 minutes. Add fresh thyme. Remove from heat and set aside.
In saucepan, melt butter and add flour. Cook over medium heat and stir constantly for 2 minutes. Pour milk and cream into large measuring cup. Slowly add milk mixture 1/4 cup at a time, constantly. Increase heat to medium high, and bring mixture to boil. Add nutmeg, mustard powder, and season with salt and pepper. When mixture is bubbling and has thickened, reduce heat to low, add 3/4 cup blue cheese and all but 2 tablespoons parmesan, leaving some for the top.
Add cooked pasta to sauce and mix well. Add the reserved pancetta. Check seasonings. Add more salt and pepper as needed. Pour pasta into a 2 quart casserole dish and cover with Panko crumbs. Dot the top with remaining 1/4 cup blue cheese and 2 tablespoons parmesan. Bake 20 minutes, or until golden brown and bubbly.
Top with fresh parsley.
Slowly adding the milk and cream guarantees the smoothest sauce possible, making a rich and decadent baked pasta dish.