I first season both sides of the chicken thighs with salt and pepper. Then I heat olive oil in a skillet or Dutch oven over medium-high heat. Once the oil is hot, I brown the chicken on one side (4-5 minutes), nestle onions between thighs so they soak up juices, then flip chicken and brown the other side.
After browning, I move chicken and onions into the slow cooker. I sprinkle Italian seasoning, garlic powder, onion powder, then pour in chicken broth. I added pepperoncini (with brine) and drained, rinsed giardiniera. Then I cover and cook: about 3-4 hours on high, or 6-8 hours on low. When done, I remove chicken, shred with two forks, and put it back in to soak up the juices.
I preheat the oven to 375°F and split the hoagie buns, laying them on a baking sheet to toast about 5 minutes until the edges are golden. I place provolone on bottom halves of buns so the cheese melts from warm bun + hot chicken. Then I pile on shredded chicken, topped with extra giardiniera and pepperoncini slices. Close the sandwich and serve hot.