Boil the chicken breasts until done then debone discarding bones and skin. Set aside. Add noodles to the broth from the chicken and cook. Meanwhile, saute' onions in butter seasoned with salt and pepper. Add onions, pimentos and broth bringing to a light boil then turn the heat down and add cubed velvetta a little at a time until melted then adding chicken. Add more salt & pepper if needed. In large roaster, add spaghetti and sauce. Stir together and bake approximately 15 minutes at 350 degrees.