Thinly slice 1 medium red bell pepper (about 1 cup) and dice ½ medium yellow onion (about ¾ cup). Thinly slice 10 whole pepperoncini peppers into rings (about ½ cup), or measure out ½ cup sliced pepperoncini. Finely grate 1 ounce Parmesan cheese on the small holes of a box grater (about ½ cup), or measure ⅓ cup store-bought grated.

Cut 1 ½ pounds boneless, skinless chicken breasts across the grain into ½-inch-thick slices. Place in a large bowl, add ¾ teaspoon of the kosher salt and ¼ teaspoon black pepper, and toss to coat.

Heat 2 tablespoons of the olive oil in a large skillet over medium-high heat until shimmering. Add half of the chicken in a single layer and cook until deeply browned and cooked through, about 6 minutes total. Using tongs, transfer to a plate. Repeat with the remaining chicken.

Add the remaining 1 tablespoon olive oil to the skillet. Add the bell pepper, onion, remaining ½ teaspoon kosher salt, and 2 teaspoons Italian seasoning. Cook until beginning to brown, about 2 minutes. Add ½ cup low-sodium chicken broth and scrape up the browned bits from the bottom of the pan. Cook until the broth is reduced by half, about 2 minutes.

Add the Parmesan and ½ cup heavy cream. Bring to a simmer and cook until slightly thickened and the peppers are tender, about 3 minutes. Return the chicken and any accumulated juices to the pan. Add the pepperoncini and toss to coat in the sauce. Serve with more Parmesan cheese if desired.