Place chicken in a large saucepan and add just enough water to cover (I use a 4-quart dutch oven or stock pot). Bring to boiling on medium-high heat, then reduce the heat. Cover the pan and simmer until the chicken is no longer pink (about 15-20 minutes).

Remove chicken. When cool enough to touch, cut into bite - size pieces.

Skim/strain water in saucepan, reserving about 1 1/4 cup - 1 1/2 cup. Rinse/wipe out saucepan if necessary, and return the reserve water (now broth) to the saucepan.

Melt butter in skillet over medium-high heat. Add onion and celery. Saute until good and tender. Add to saucepan.

Add Velveeta cheese to saucepan (or, optionally, cream of chicken soup). Simmer over medium to medium-low heat until Velveeta has melted through, stirring often.

Add chicken, pimentos, Rotel, and any other optional ingredients to saucepan and stir well. I usually chop up and add a drained 6.5 oz can of mushrooms and 1 cup of drained, canned sweet peas.

Add cooked spagetti and mix well.

Pour into a 4-quart baking dish and sprinkle liberally with shredded cheese.