Prep your vegetables.
Slice the onion and peppers, prepare the mushrooms if using, and mince the garlic. This is where your kitchen starts smelling wonderful!
Build the sauce.
Mix the crushed tomatoes, tomato sauce, chicken broth, garlic, Italian seasoning, oregano, basil, paprika, pepper, and red pepper flakes in the crockpot.
Add the chicken and vegetables.
Nestle the chicken breasts into the sauce. Arrange the peppers, onions, and mushrooms around and over them. Stir the vegetables gently into the sauce. Add salt conservatively, then adjust at the end.
Let the crockpot do its thing.
Cook covered for approximately 3-4 hours on LOW or 2-3 hours on HIGH.
Timing varies with the thickness of the chicken and your appliance.
Check the thickest part with a food thermometer; chicken must reach.
165°F.
Finish that beautiful sauce.
Remove the chicken and slice it or shred it into large pieces. Stir in the balsamic vinegar, if using, taste, and adjust the seasoning. Return the chicken to the sauce. If the sauce is too thin, remove the lid and cook on HIGH for 15-20 minutes, watching the consistency.
At the end add in your tortellini just until warmed through or separate.
For deeper flavor: Sauté the onions and peppers in a skillet for a few minutes before adding them.
You can also brown the chicken briefly, although neither step is required.
For a richer Italian sauce: Add a handful of sliced black olives or a tablespoon of capers near the end.