Thinly slice a large yellow or sweet onion and place in the bottom of the Crock-Pot.
Trim excess fat from 6 boneless, skinless chicken thighs. It is not necessary to trim all the fat from the chicken, just any large pieces.
Place the chicken on top of the onion slices. Sprinkle with garlic salt, black pepper, red pepper flakes, Italian seasoning, and dried basil.
Drizzle seasoned chicken with olive oil and melted butter.
Pour the jar of spaghetti sauce over the chicken. Pour in water.
Place the lid on the Crock-Pot and cook fully thawed chicken on high for about 3 hours or low for about 6 hours, until the thickest part reaches at least 165°F on a food thermometer and the chicken is tender. Keep the lid on except when checking doneness.
After the chicken reaches 165°F and is tender, optionally turn the Crock-Pot to high and remove the lid for 30–60 minutes to evaporate excess liquid, if your appliance instructions allow. This optional reduction adds time.
Shred the chicken with 2 forks.
Taste your Crock-Pot Italian chicken and adjust the seasonings. You may want to add more herbs, garlic salt, or pepper.
Serve over your favorite pasta, or over roasted or steamed vegetables.
Garnish with grated Parmesan and/or chopped fresh basil, if desired.