Preheat oven to 200 degree Celsius.
Grease butter on baking dish

Cook pasta in boiled water until cooked, about 8 mins. Drain well and reserve.

In the same saucepan, melt butter over medium-high heat.
Stir in milk, flour, salt, pepper, and onion until smooth.
Cook for 2 minutes.

Reduce heat to low, whisk in 1/3 cup cheese and hot sauce.
Stir in cooked pasta.

Transfer to prepared bakeware.
Sprinkle cheese on top of pasta.

Bake uncovered until cheese is melted, about 10 minutes.