Preheat oven to 350 degrees F (175 degrees C). Sprinkle flour into oven bag and place it in a 9x11-inch pan.
Spread onion, celery, and carrot pieces on the bottom of the bag. Rinse turkey thighs and pat dry with paper towels. Rub olive oil under the skin and on the outside of the thighs.
Sprinkle seasoned salt liberally on turkey thighs and rub it under the skin (see Cook's Note). Arrange thighs on top of vegetables. Close the bag with the tie. Make six slits in the top of the bag with a sharp paring knife. Place pan in the oven, making sure oven bag does not touch top heating element.
Roast until an instant-read thermometer inserted into the thigh meat reaches 165 degrees F (74 degrees C), about 1 hour.
