Cut the chicken breast into rough cubes, of about 1 inch. Finely chop the mushrooms and onions.
Over medium high heat, sautee the mushrooms and onions in the butter until soft. Add the chicken, garlic salt, and pepper, and mix well.
In a large pot of boiling water, boil the spaghetti for about 8 minutes or until done a little less than your liking, as it will cook a bit further in the oven. Drain completely.
Chop the block of Velveeta into approximately 1 inch cubes.
In a large casserole dish, combine the soups, drained can of Ro-Tel, velveeta, and chicken mixture. Add the pasta and mix together well. (Be mindful of mixing up the velveeta because sometimes the little cubes like to clump together, expecially after getting a little warm from the pasta.).
Bake covered in a 350 degree oven for 30 minutes. I like the top of mine crispy, so I take off the cover and bake for another 10 minutes. If you don't like crispy, then keep it covered.
Be sure to let it cool for about 10 mintues prior to serving.