Preheat oven to 425 degrees.
Peel and cube potatoes and carrots into bite size (about 1 inch) pieces. Coarsely chop onion into thick pieces.
In a large cast iron skillet over medium heat, add 1 tablespoon olive oil to pan. Season turkey tenderloin with poultry seasoning, salt, and pepper. Sear tenderloin for 2-3 minutes on each side, until lightly browned. Remove from heat.
Add 1 tablespoon olive oil, cubed potatoes, carrots, and onions to skillet surrounding your turkey tenderloin. Toss to coat in pan drippings. (If you don't have enough room in your skillet, transfer turkey tenderloin and vegetables to a baking sheet instead.)
Bake for 20-25 minutes until turkey tenderloin is cooked through and vegetables are fork tender. Time will vary based on the thickness of your tenderloin.
Remove turkey and vegetables from skillet. Allow turkey to rest for 5 minutes before slicing and serving.
Meanwhile, add butter to skillet over medium heat. When butter has melted and begins to bubble, add flour and stir to combine. Cook for 1 minute. Add chicken broth and whisk constantly until fully combined. Season with salt and pepper and allow gravy to reduce for 4-5 minutes or until thickened.
Serve gravy on the side or poured over your turkey tenderloin.