In a spice grinder or mortar, combine kosher salt, caraway seeds, sage, fennel, coriander, and rosemary. Grind to a coarse powder. Transfer spice mixture to a bowl and stir in paprika, garlic powder, flour, and onion powder; mix in vegetable oil to make a smooth paste.
Pat chicken halves dry with paper towels and tuck wing tips up behind the back. Brush spice paste onto chicken halves, coating both sides, taking care to season under wings and legs. Place chicken halves in baking dish or roasting pan with skin sides up, leaving space around chicken so halves aren't touching.
Roast in preheated oven until a thermometer inserted in a thigh reads 165 degrees F (74 degrees C), about 1 hour. Remove from oven and let rest for 10 minutes before slicing.