Slice onions, bell peppers, and mushrooms. Slice chicken breasts into thinner slices if they are super thick.
Add crushed tomatoes, chicken broth, white wine, garlic, sugar, salt and pepper to slow cooker. Mix to combine.
Add slices of chicken breast to slow cooker. Then add onions, bell pepper, mushrooms, thyme and bay leaf. Use a spoon to submerge the ingredients into the sauce as much as possible (okay if not fully submerged).
Cook on high for 3.5-4 hours.
Taste and add more salt if needed. Salt will open up the flavors of the sauce.
Remove bay leaf, thyme and garlic pieces. Serve chicken with (lots of) sauce over a bed of rice. Enjoy!