Preheat oven to 375 degrees.
Boil water in a big pot, season generously with salt. Cook pasta as per package instructions. Pasta should be cooked just 'al dente'. Reserve 1 cup of cooked pasta water for later use.
While the pasta is cooked, melt butter in another in a large skillet/pot.
Stir in flour. Let it bubble on low heat for about 30-40 seconds. The raw smell of the flour should be gone but make sure the flour doesn't get brown.
Slowly add half of the milk to the skillet while stirring the mixture continuosly with the other hand.
The mixture will start to thicken quickly. Then slowly add the rest of the milk and keep stirring to avoid any lumps.
Reduce the heat. Add half and half, 1 cup mozzarella cheese, parmesan cheese, yellow mustard powder, herb seasoning, garlic powder, onion powder , salt and pepper. Mix everything well.
Drain tuna well and break any big chunk to medium sized pieces. Add it to above sauce along with cooked pasta.
After mixing if it looks too thick, then add about ½ cup pasta water saved to the skillet to get a creamy consistency.
Spray a 9X13 casserole dish with non stick spray. Pour in the pasta mixture to the dish.
Bake casserole for 25 to 30 minutes, uncovered, or until heated through and the edges start to set.