Preheat the oven to 425 degrees F (220 degrees C).

Combine kosher salt, caraway, sage, fennel, coriander, and rosemary in a spice grinder or mortar. Grind to a coarse powder. Transfer spice mixture to a bowl; 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 behind the back. Brush spice paste onto chicken halves, coating all sides and seasoning under wings and legs. Place chicken halves, skin-sides up, in a baking dish or roasting pan, leaving space around chickens so halves aren't touching.

Roast in the preheated oven until no longer pink at the bone and the juices run clear, about 1 hour. An instant-read thermometer inserted into the thickest part of the thigh, near the bone, should read 165 degrees F (74 degrees C). Remove from the oven and let rest for 10 minutes before slicing.