Preheat oven to 375°F. Place a rack in a large roasting pan. Remove excess skin and fat from turkey. Remove and discard neck and giblets. Rinse turkey under cold water and pat dry. Season inside of cavity with salt and pepper. Place onion and lemon inside cavity. Tie legs together and tuck wings under.
Melt butter with oil in a small pan over medium high heat. Stir in thyme, oregano and tarragon. Place turkey on rack in roasting pan. Brush with herb mixture. Use your hands to spread and rub mixture into and under skin. Pour 1 cup water into bottom of pan. Reduce oven temperature to 325°F; roast turkey until a meat thermometer inserted into thickest part of thigh reads 170°F, about 4 hours. Baste every 20 to 30 minutes with juices from bottom of pan.
Remove turkey from oven, place on a cutting board and cover loosely with foil; let rest for 20 to 30 minutes. Pour pan juices into a gravy separator or bowl. Skim fat that rises to top, reserving 1/4 cup fat and skimmed pan juices separately. Pour reserved fat into roasting pan, set pan on stove over two burners and turn heat to medium-high. Sprinkle flour over fat and cook, stirring, for 2 minutes. Mixture will be dry and lumpy. Pour in broth and whisk to blend. Strain.
Pour pan juices into gravy. Bring to a boil and whisk vigorously until thickened and smooth. Season with salt and pepper
Carve turkey and serve gravy on side.