In a saucepan over medium heat, melt the butter.
Add in the garlic and stir for about 1 minute.
Whisk in the flour (whisking continually for 1 minute so that it does not burn).
Slowly pour in the chicken broth, whisking as your pour it in, and stir for a few minutes until the sauce has thickened up.
Remove the saucepan from the heat, and stir in chili powder, cumin, garlic powder, salt, and black pepper; set aside.
Spray the slow cooker with nonstick cooking spray.
Place the chicken breasts in the bottom of the slow cooker.
Top with diced tomatoes, jalapeño, bell peppers, and onion. Pour the sauce over the top.
Cover the slow cooker with the lid and cook on low for 4 to 6 hours (or high for 2-3 hours), making sure to not overcook the chicken.
Transfer the chicken to a cutting board and stir the cheese into the slow cooker.
Using 2 forks, shred the chicken and add it back into the slow cooker, stirring everything together.
Let the slow cooker cook on low heat for 15 minutes, allowing the cheese to melt.
Serve topped with fresh cilantro, if desired.