Philly Cheesesteak Stuffed Peppers

Preheat oven to broil with rack about 6 inches from heat. Cut bell peppers in half lengthwise; remove and discard seeds and membranes. Toss together halved bell peppers, 1 tablespoon of the oil, and 1/2 teaspoon of the salt in a 13- x 9-inch broiler-safe baking dish. Arrange bell peppers cut side down (they can overlap slightly). Broil in preheated oven until bell peppers are blackened in spots and starting to soften, 5 to 8 minutes. Remove from oven. Flip bell peppers over, and return to oven. Broil until tender-crisp, 6 to 8 minutes. Remove from oven; set aside. Turn oven off.

Heat 2 tablespoons of the oil in a large nonstick skillet over medium. Add onions, 1 teaspoon of the salt, and 1/2 teaspoon of the black pepper; cook, stirring occasionally, until onions are translucent and starting to brown, about 20 minutes. Reduce heat to medium-low; cook, stirring occasionally and keeping onions in an even layer, until very tender and brown throughout, about 20 minutes. Remove from heat; stir in vinegar. Transfer to a plate; cover to keep warm. Do not wipe skillet clean.

Add 1 tablespoon of the oil to skillet; heat over high. Sprinkle steak evenly with remaining 1 teaspoon each salt and black pepper. Add steak to skillet; cook until desired degree of doneness, about 3 minutes per side for medium rare (about 145°). Transfer to a cutting board; let rest 10 minutes.

Meanwhile, preheat oven to broil with rack about 6 inches from heat. Toss together breadcrumbs, parsley, and remaining 1 tablespoon oil in a small bowl.

Slice steaks against the grain as thinly as possible. Place 1 cheese piece in cavity of each bell pepper half, top evenly with steak, onions, and remaining cheese pieces. Sprinkle tops evenly with breadcrumb mixture. Broil in preheated oven until cheese is melted and breadcrumbs are toasted, 2 to 3 minutes. Garnish with additional parsley.