Place the diced onion in a large pot of lightly-salted water; bring to a boil. Cook the pasta until al dente, 8 to 10 minutes; drain. Transfer the onion and pasta to a casserole dish; season with salt, pepper, and garlic salt. Stir the butter and brown sugar into the pasta mixture until the butter melts. Add the Cheddar cheese soup, Havarti cheese, Parmesan cheese and 1/3 cup of Cheddar cheese; stir well. Sprinkle remaining Cheddar cheese over top of the dish.
Bake in preheated oven about 45 minutes. Change oven setting to 'Broil' and cook until top is golden brown, about 4 minutes.
