Season chicken breasts with a little salt and pepper, place in medium frying pan with a little oil.
Add chopped onions.
Cover with tight lid and cook breasts over medium-low heat til done, about 12-18 minutes depending on their size. Turn breasts and stir onions occasionally to prevent browning. Set aside to cool when done.
In small pot, cook rotini noodles for 8 minutes, drain.
Meanwhile, in large bowl, combine soups, cheese and mayo. Microwave til melted, stirring occasionally, about 5 minutes.
Cut up chicken and add chicken and onions to cheese mixture.
Fold in noodles, broccoli, and mushrooms.
Pour into greased 13x9-inch pan or casserole dish.
Bake in 350ºF oven for 20 minutes.
Top with cheddar fried onions, and bake 5 minutes longer.