Place chicken, onions, bell peppers, and garlic in the bottom of a slow cooker. Drizzle with olive oil; toss to coat. Season the chicken and veggies liberally with salt and pepper to taste. Sprinkle with Italian seasoning. Toss to coat everything in the seasoning.
Cover and cook on LOW for about 4-6 hours, or on HIGH for about 2-3 hours.
Remove the chicken to a cutting board. Slice the meat or shred with two forks. Return the chicken to the slow cooker and stir it into the vegetables.
Arrange the cheese slices on top of the chicken mixture. Cover with the lid and continue cooking just until the cheese melts, about 3-5 minutes.
Serve the chicken, peppers, onions, and cheese on the toasted hoagie rolls.
Notes
Season the meat and vegetables liberally with salt and pepper. Taste and add more when it's done. That's a lot of meat and veggies, so it requires a good amount of seasoning so that it doesn't taste bland.
Be careful not to overcook your chicken. I find that the chicken breast or thighs have the best texture when cooked in my slow cooker on HIGH for about 2-3 hours (at the most -- sometimes they're done in just 90 minutes). That keeps them moist and juicy, and prevents them from drying out. The meat is done when it reaches an internal temperature of 165°F.
Also remember — every slow cooker runs at a slightly different temperature (which is why the recipe gives you general time ranges, rather than exact times). Since my slow cooker tends to cook at a relatively high temperature, I find that the meal is best when cooked on HIGH for about 2 hours or on LOW for about 4 hours. Just get familiar with your pot and adjust accordingly.
Use cheddar cheese, Colby cheese, Monterey Jack, Swiss, mozzarella, or American cheese instead of the provolone.
Instead of serving the chicken and vegetables in toasted hoagie rolls for a chicken cheesesteak sandwich, put the meat on rice bowls, over salads, in wraps, or in baked potatoes.
Use either boneless, skinless chicken breasts or boneless, skinless chicken thighs for this recipe. We find that the dark meat thighs stay a bit juicier during the slow cooking process, but they are both great.