Preheat your oven to 450°F with the top rack positioned about 4–6 inches from the broiler.
Place the potatoes in a large pot and cover with cold water. Season the water generously with salt. Cover and bring to a boil over high heat. Once boiling, remove the lid and cook for 5–10 minutes, or until the potatoes are fork tender.
Drain the potatoes and pat them dry with a paper towel.
While the potatoes are cooking, combine the melted butter, minced garlic, Kinder’s Cowboy Butter seasoning, and Worcestershire sauce in a bowl. Stir well and set aside.
Spray a large sheet pan (preferably 21 x 15-inch) with cooking spray.
Arrange the potatoes, green beans, sliced red bell pepper, and sliced onion on the sheet pan. Drizzle half of the cowboy butter over the vegetables and gently toss to coat evenly. Season with salt and pepper.
Roast the vegetables for 20 minutes. They should be tender but not fully caramelized yet.
Pat the flat iron steaks dry and season both sides generously with salt and freshly ground black pepper.
Place the steaks over the roasted vegetables and drizzle with the remaining cowboy butter.
Turn the oven to the broil setting and place the sheet pan under the broiler. Broil for 4 minutes, then flip the steaks and broil for another 3–4 minutes for medium-rare.
Remove the pan from the oven and let the steaks rest for 5–10 minutes before slicing and serving.